Normally, when a track is looped, there's no time between repetitions of the audio. Sometimes this is exactly what you want, but in other cases you might want to make your mix less "busy". The Auto-Trigger feature allows you to do this.
Middle-click (or Ctrl+left-click) over the position slider, to display the Auto-Trigger dialog. In the dialog, press the play button. The dialog closes, and the track now plays every five seconds (or whatever automation time you selected), regardless of its length. If the track is shorter than the automation time, the remaining time is silent. If the track is longer than the automation time, the track gets cut off.
Auto-Trigger works in combination with creating a selection. In other words, you can use the selection feature to create a "sample" within an audio file, and then use Auto-Trigger to play that sample repeatedly at some fixed time interval.
While a track is auto-triggering, left-clicking on its position slider doesn't set the audio position as usual; instead, it immediately triggers the audio, and resets the triggering loop. This allows you to synchronize the track, relative to other tracks.