If you don't understand, don't hesitate to ask. Some Chinese seem to feel that it is impolite to ask questions even when they don't quite understand. However, it's much better to say that you don't understand than to pretend you understand and answer very vagely. When you don't understand, or have a clock what was said, you may ask by saying any of the following.

I beg your pardon? I am not clear on what you said.

I am afraid I don't understand you. Would you mind saying it again?

I am sorry I don't follow you. Could you repeat that?

I didn't get it.

You'll improve your understanding and the speaker will be happy to make his point again.