When you do not understand what someone has said, please, please, please, never say
What?. This is especially true when you are with strangers or in formal and business situations. It might be OK to reply with a What? when speaking in your language but in English, it is a no-no. It sounds extremely rude even if it is not your intention to be impolite.
Here are some possible, polite responses:
I didn’t get that.
Can you repeat, please?
Believe me when I say that after 30 years of teaching English to foreigners, hearing someone say
What? still makes me wince!