Often used in an insulting way when an unintelligible comment has been made.
Can however be used when you've missed some piece of information and need to be informed.
1) Guy 1: Even though my place was locked I suspect that someone broke it, clubbed me over the head, spread beer bottles around me, and then pissed on my playstation 2.
Guy 2: Hold the phone! There's a playstation 2?
2) Guy 1: So Cindy asked me what my job was after we did it, and I asked her why she was still here.
Guy 2: Whoa, whoa, hold the phone, how'd you hook up with her?