"I don't understand what do you mean?"就是个典型的what引导的宾语从句。
顺便说一下，你给的例子是个what引导的定语从句。
宾语从句指因为宾语很长或是一个句子而用what/that等连接的句子，如I don't understand what do you mean?中的what do you mean。定语从句指用what/that/which/who等对于主语、宾语的补充和解释。
what可以引导宾语从句吗?WHAT可以用来引导非限定宾语从句吗"I don't understand what do you mean?"就是个典型的what引导的宾语从句。
顺便说一下，你给的例子是个what引导的定语从句。
宾语从句指因为宾语很长或是一个句子而用what/that等连接的句子，如I don't understand what do you mean?中的what do you mean。定语从句指用what/that/which/who等对于主语、宾语的补充和解释。
与what有关的宾语从句The manager came up to see ________
A. what was the matter
相似题：He was anxious to know A.what was the matter
记住朋友：what was/is the matter ;what is wrong;what is trouble等固定巨型 都用疑问句语序，其他的就用陈述句语序，what 作主语，was作谓语,the matter作宾语。
就是一个固定的巨型在从句中都用原来的语序，还有 I dont know what's the matter 都是一样的当你看到从句的主谓宾全了就选that。取消句子独立性。如I like the girl that she is fat
当缺少成分时如 I like the box ___ is blue .. 这事就用what
在定于从句中 what/that 用于物 which/that 用于人。。。而宾语从句只要看却不缺成分就可以了解释的已经很清楚了呀..Sth is the matter 这里sth不知道所以用what代替．这是宾语从句，从句里面语序不发生变化．说详细点The manager came up to see what was the matter意思就是这个经理走上前看究竟发生了什么．看的东西用一句句子表达了出来．就是what was the matter发生了什么，这个”什么”在what was the matter里就是what．当然是主语．一般过去时态：表示过去某一时间所发生的动作或存在的状态。谓语动词要用一般过去式。经常与yesterday（昨天）, last week（上周）, last month（上个月）, last year（去年）, two months ago（两个月前）, the day before yesterday（前天），in 1990 (在1990年), in those days （在那些日子里）等表示过去的时间状语连用。
如： I was born in 1990. (我出生在1990年)。
When did you go to the park? （你是什么时候去的公园）。
I went to the park last week. （我是上周去的公园）
在上面的句子中**句属于be动词的一般过去时态；第二句和第三句属于实义动词的一般过去时态。
1． Be 动词的一般过去时态
在没有实义动词的句子中使用be动词， am is 的过去式为was; are的过去式为were.
构成：肯定句：主语+was (were) +宾语
如：I was late yesterday. （昨天我迟到了。）
否定句：主语+was (were) +not+宾语
如：We weren't late yesterday. (我们昨天没迟到)
疑问句：Was (Were) +主语+宾语
如： Were you ill yesterday? （你昨天病了吗？）
肯定回答： Yes, I was. (是的，我病了。)
否定句： No, I wasn't. (不，我没病。)
特殊疑问句： 特殊疑问词+was (were) +主语+宾语
如：When were you born? 你是什么时候出生的？
由which和what引导的宾语从句的区别1、I want to know what book you like to read.
2、I want to know whick book you like to read best of the three.
在上面这两句话种，可以很清楚的看到what ＆which的区别
what 没有范围
which 是在有范围之内的一种选择
宾语从句
一、定义：
用作宾语的从句叫宾语从句.宾语从句可以用连词that,连接代词who,whom,what,which；连接副词 how,when,where以及if和whether引起.连接代词和连接副词在宾语从句中作一相应的句子成分,if或whether表示疑问,而that没有意义仅起连接作用.
二、宾语从句的分类：
1． 作动词的宾语从句：
如：Everyone knows that he is a good student.
He wondered how the pyramids were built.
2． 作介词宾语：
如：This depends on how hard you work.
Is there anything wrong in what I said?
3． 作形容词的宾语：
如：They are confident that they can do the job well.
I am not certain whether the train will arrive on time.
三：使用宾语从句要注意的问题：
1． 宾语从句引导词that的省略：
在非正式场合下,that在引导宾语从句时,可以省略.
如：I think (that) you are right.
2． 形式宾语it：
如果宾语从句后面跟有补语,要用形式宾语it来代替,而将从句放到补语的后面去.如：He has made it clear that the meeting will not be postponed.
I heard it said that that factory was founded in 1901.
3．宾语从句的时态呼应：
如果主句的谓语是过去时,宾语从句的时态要按照时态呼应规则进行相应的调整.但是若宾语从句表示的是客观真理或自然观念,其谓语时态仍用一般现在时.
如：He said that he had left his umbrella in the library.
The teacher told the students that the Pacific Ocean is the largest ocean in the world.
4． 宾语从句否定意义的转移：
在think, believe, suppose, expect 等动词所根的宾语中,如果从句谓语是否定的,一般要将否定词not转移至主句谓语上去,而将从句宾语变为肯定形式.
如：I don’t think he has time to play chess with you.
I don’t suppose it is the rush hour yet.
5．一些动词接的宾语从句通常要用虚拟语气：
一般说来,在一些表示坚持(insist),命令(order,mand),建议(suggest,advise),要求(require,demand) 等动词之后的宾语从句中,谓语一般要用“should +动词原形”（其中的should在美国英语中常省略,但是引导从句的that通常不省略）：
如：She suggested that we (should) leave early．
He ordered that the medicine (should) be sent by a special plane．
6．whether 与 if
引导介词宾语从句时,只能用 whether,不用if；与“or not”连用时,一般用whether,不用if.
如：I am interested in whether he'll go abroad．
We don't know whether he will e or not．
7．宾语从句的语序
连接词后面为陈述语序.
如：I don’t know what your name is.
8．直接引语变为间接引语
间接引语的使用应注意选择合适的动词.陈述用say／tell,疑问用ask／wonder,而祈使用 tell／order,建议用advise／suggest.注意这些动词后面的结构和语气.
如：“Could you get some tea for me?” Mr．Blake said to his wife．
→Mr. Blake asked his wife whether she could get some tea for him.
“Shall we go dancing tonight?”John said．
→John suggested they should go dancing tonight.
状语从句
状语从句是句子的状语由一个从句充当,来修饰主句中的动词,形容词或副词等.状语从句
都由从属连词引导,与主句连接,放在句末时,一般不在前面加逗号.
状语从句根据它表示的意思可分为时间,原因,条件,比较,结果,目的等类.下面我们拣重点
的一个一个来分析.
时间状语从句：是由when, as, while, after, before, since, until, as soon as 等从
属连词引导的状语从句.
时间状语从句中的谓语动词不能用一般将来时,只能用一般现在时表示将来发生的动作或
存在的状态.
如:I will call you as soon as I arrive there.
原因状语从句： because, since, as和for都表示原因.常常令我们不知该用哪个好.我们来
比较一下.
because语势最强,回答why提出的问题,用来说明人所不知的原因.当能够很明显的看出原因
或人们已知原因,就用as或since.
如：I don't like that coat,because the color looks terrible.
由because引导的从句如果放在句末,且前面有逗号,则可以用for来代替.但如果不是说明直
接原因,而是多种情况加以推断,就只能用for.
如：He is not here, because / for his mother is ill.
目的状语从句：表示目的状语的从句可以由in order that, so that,等词引导.如：
You must raise your voice so that/in order that everybody can hear you clearly.
结果状语从句：结果状语从句常由so...that 或 such...that引导,要掌握和区分这两个句型,
首先要了解so和such后面分别跟什么词.such是形容词,修饰名词或名词词组,so是副词,只能修
饰形容词或副词.so 还可与表示数量的形容词many, few, much, little连用,形成固定搭配.
如：The box is so heavy that I can't carry it.
让步状语从句：是由though, although 引导的状语从句.
though, although 和 but不能同时使用.
Although it rained, they had a good time.
定语从句
定语从句（Attributive Clauses）在句中做定语,修饰一个名词或代词,有时也可以修饰部分或整个句子.
被修饰的名词,词组或代词即先行词.定语从句通常出现在先行词之后,由关系词（关系代词或关系副词）引出.
关系代词有：who, whom, whose, that, which,as .
关系副词有：when, where, why ,how .
关系代词和关系副词放在先行词和定语从句之间,起连接作用,同时又可做定语从句的一个成分.当关系代词做宾语时可以省略.
定语从句中的谓语动词必须在人称上和数量上和先行词保持一致.
定语从句分为限制性定语从句和非限制性定语从句.
1 、关系代词引导的定语从句
1）who, whom, that
这些词代替的先行词是人的名词或代词,在从句中所起作用如下：
Is he the man who/that wants to see you?（who/that在从句中作主语）
He is the man whom/ that I saw yesterday.（whom/that在从句中作宾语）
2) Whose 用来指人或物,(只用作定语, 若指物,它还可以同of which互换),
They rushed over to help the man whose car had broken down. Please pass me the book whose (of which) cover is green.
3)which, that
它们所代替的先行词是事物的名词或代词,在从句中可作主语、宾语等,例如：
A prosperity which / that had never been seen before appears in the countryside.（which / that在句中作宾语）
The package (which / that) you are carrying is about to e unwrapped. (which / that在句中作宾语)
关系代词that和which 都可以指物,that 和Who 都可以指人,其用法区别：
1）不用that的情况
a) 在引导非限定性定语从句时
(错) The tree, that is four hundred years old, is very famous here.
b) 介词后不能用
We depend on the land from which we get our food.
c)多用who 的情况
①关系代词在从句中做主语
A friend who helps you in time of need is a real friend .
②先行词为those , people 时
Those who were either fools or unfit for their offices could not see the cloth .
③先行词为all, anyone , ones , one 指人时
One who doesn’t work hard will never succeed in his work .
④在There be句型中
There is a stranger who wants to see you .
⑤在被分隔的定语从句中
A new teacher will e tomorrow who will teach you German .
⑥在有两个定语从句的句子中,其一用who,其二用that,但若先行词后接两个以上的并列定语从句时,后一个必须重复前一个关系代词.
The student who was praised at the meeting is the monitor that is very modest and studies very hard .
There is a teacher who is always ready to help others and who enjoys what he does .
2) 只能用that作为定语从句的关系代词的情况
a) 在不定代词,如：anything, nothing, everything, all, much, few, any, little等作先行词时,只用that,不用which.
All that is needed is a supply of oil.
Finally, the thief handed everything that he had stolen to the police.
b) 先行词有the only, the very, the just修饰时,只用that.
He is the very man that helped the girl out of the water .
c) 先行词为序数词(the last)、数词、形容词最高级时,只用that..
The first English book that I read was “The Prince and the Pauper” by Mark Twin .
d) 先行词既有人,又有物时.
He talked about the teachers and schools that he visited .
e)当主句是以who 或which 开始的特殊疑问句时,用that 以避免重复.
Who is the person that is standing at the gate .
f)关系代词在从句中做表语
He is not the man that he used to be .
2 、关系副词引导的定语从句
关系副词可代替的先行词是时间、地点、方式或理由的名词,在从句中作状语.
1)when, where, why,how
关系副词when, where, why,how的含义相当于"介词+ which"结构,因此常常和"介词+ which"结构交替使用,例如：
There are occasions when (on which) one must yield.
Beijing is the place where (in which) I was born.
Is this the reason why (for which) he refused our offer?
I’m surprised the way how (by which) he works out the problem .
①在非限制性定语从句中,"介词+ which"结构不能代替关系副词.
如：They set up a state for their own , where they would be free to keep Negroes as slaves .
②含有介词短语的动词一般不能拆开,介词仍放在动词后面.
Is this the book which (that) she was looking for ?
3、名词/数词/代词 /形容词最高级 + 介词 + 关系代词引导定语从句
She has written a book , the name of which I have forgotten .
There are fifty-five students in our class , all of whom are working hard .
There are five continents in the world , the largest of which is Asia .
4、 as, which 引导非限定性定语从句的差别：
由as, which 引导的非限定性定语从句,as和which可代整个主句,相当于and this或and that.As一般放在句首,which在句中.
As we know, **oking is harmful to one's health.
The sun heats the earth, which is very important to us.
as可引导非限制性从句,常带有'正如'的意思.
As is know, **oking is harmful to one's health.
用法区别：
(1) as 引导的定语从句可置于句首,而which不可.
As we all know , he never **okes .
(2) as 代表前面的整个主句并在从句中作主语时,从句中的谓语必须是系动词；若为行为动词,则从句中的关系代词只能用which..
(3)非限定性定语从句中出现expect , think , suppose 等表示猜测、想象、预料等时.
She succeeded in her doing the research work , as we expected .
(4)As 的用法 the same… as；such…as 中的as 是一种固定结构, 和……一样…….
I should like to use the same tool as is used here .
We should have such a dictionary as he is using .which和what在宾语从句中同属连接代词，在从句中可做主语、宾语及定语。
它们们二者最主要的区别在于本身意义的不同，which翻译成“哪个/件/条……"；what
翻译成”什么“。而在具体用法上没有什么明显的区别，以下用例子证明。
1.which引导的宾语从句，其本身在从句中所充当的成分。
I don't know which **ke belongs to my daughter.（做定语）
我不知道那辆自行车是我女儿的。
He asked me which I liked best.（做宾语）
他问我喜欢哪本书。
She wants to know which is the best.(做主语）
她想知道哪个是最好的。
2.what引导的宾语从句，其本身在从句中所充当的成分。
I wonder what he does.（做宾语）
我想知道他是做什么工作的。
I don't know what happened.(做主语）
综合比较之后，可以看出它们的区别主要是词义方面的。which和what在宾语从句中同属连接代词,在从句中可做主语、宾语及定语。 它们们二者最主要的区别在于本身意义的不同,which翻译成“哪个/件/条……";what 翻译成”什么“。而在具体用法上没有什么明显的区别,以下用例子证明。 1.which引导的宾语从句,其本身在从句中所充当...”which的意思是哪一个，后面还要接名词。what相当于which接名词，
意思是which是指，哪一个东西（后面的名词所指的事物。）what着有“所...”的意思，一般指比较抽象的概念的事物。
I wanted to know which school it was you went to.
我想知道你上的是哪所学校。
He drinks what is left in his glass.
他把杯子里所剩下的东西。