You are letting someone know that you are not sure about what is being asked. You may also have no knowledge or opinion on a topic.

你让别人知道你不确定所问的是什么，你也可能对某个话题一无所知。

Here are some examples: 这里有一些例子： 

"I don't know what to eat for dinner."

 我不知道晚餐吃什么。

"I don't know what to buy you for your birthday." 

我不知道给你买什么生日礼物。

"I don't know what to say."

 我不知道说什么。

"I don't know what to do with my spare time."

 我不知道怎样利用我的业余时间。

"I don't know what to do for vacation."

 我不知道假期做什么。

"I do not know what to do to make you happy."

 我不知道怎么让你开心。

"I do not know what to do to help you understand."

 我不知道怎么能帮你理解。

"I do not know what to think."

 我不知道想什么。

"I do not know what to do to prevent this."

 我不知道该做什么才能防止这种情况发生。

"I do not know what to order."

 我不知道该点什么。