Pro Tools | First Main Windows : Transport Window : Basic Transport Controls and Counters : Pre-Roll
 
Pre-Roll
During playback or record, specifies the amount of audio that plays before the Play (Timeline) Cursor or Play Start Marker location, or beginning of the Timeline selection. Pre-roll is particularly useful with punch recording since it gives you time to “catch the beat” before reaching the punch-in point. To set the pre-roll amount, type a new value in this field, or drag the Pre-Roll flag in the Main Timebase ruler.