In classes,you'd better understand as much as possible while

该句子是时间状语从句,其实句子有省略成分,还原就是：
In classes,you'd better understand as much as possible while u're talking notes.
当从句和主句的主语一致时,且从句是主语加be动词时,从句的主语和be动词可以省略.这个句子就是这种情况.
意思是课堂上,在做笔记的同时,你也要尽可能的听懂老师讲的.
while u're talking notes. 现在进行时,强调动作正在进行.take就是动词.其他例子：
while young, she was interested in math. (省略了从句的主语和be动词）
while she was young, she was interested in math
不明白的可以再问,希望帮的上你

while conj. 字典第一条释义就是 在...时, 可以改成while you are taking notes. 但是主语you出现两次, 显得重复啰嗦, 故改为 动名词, 一样的.
字典例句: She met Andy while working on a production of Carmen. 排练卡门的时候, 她认识了安迪.
They arrived while...