It's either you don't here something that somebody
said correctly and you want them to repeat it or if you yell (or put in capital
letters if you're are
writing it) the "what" it means that the other person just told you something that you don't like or unpleasantly surprised you.
1)
Friend: I can't wait for the fair
tomorrow!
You: (
busy preparing for the fair) I'm sorry, what?
2)
Boyfriend/girlfriend: I'm braking up with you.
You: I'm sorry, WHAT!!!!!!!!
Notice the
difference????