A phrase used at the moment one is speaking and realizes that they know not what the hell they are talking about, or they run into a massive stutter and must start their point over. The phrase removes what was previously said and the person talking can try again to relay the message that was previously goofed.
Jordan: So we were leaving the party and...er...uhhmm..scratch it, rewind. So we were leaving the party and this bitch says...(continues with story)
Professor Higgby: Okay class, t-t-t-tttt-tttooday. (Pauses until roars of laughter cease) Today, were going to learn Calculus.