This idiom means it is your turn to speak or do something. Read the dialog below to get a good example of how to use this expression.
Fred – So, did you do it?
Nat – Do what?
Fred – You know…
Nat – No, I don’t know.
Fred – Did you ask Carol if she likes me?
Nat – Oh, yeah, I did ask.
Fred – And?
Nat – She said she does. Now the ball is in your court.
Fred – I guess you are right. I’ll wait for her to ask me out.
Nat – No silly! You need to ask her out!