If playback is looped, this is the starting position of the loop. It's displayed either in beats and ticks, or in measures, beats and ticks, depending on the Meter property. The ending position is given by the Loop To property. The starting position must be less than the ending position, otherwise no looping occurs.
Instead of entering the loop positions numerically, you can alternatively create a step selection before using the Loop command. In that case the step selection's first and last steps determine the loop's starting and ending positions respectively. In the song grid, a loop can similarly be specified by selecting a range of cells and then using the Loop command.
If a valid loop exists, it's indicated by a highlighted area in the rulers at the top of the track steps and song grid. The loop's range can be modified by horizontally dragging the left or right edge of this highlighted area.