While Music Player is playing a track, the position indicator shows the current position inside the song. In other words: during playback, the position indicator will progress from left to right. The Position Indicator is the horizontal slider as shown below (see screenshot):
When the current track finishes, and the next track starts, the position indicator will start from its original position again (is the very left position, at 0 seconds) and move gradually to the right during playback (of the track).
The number shown directly above the position indicator is the number of seconds that have passed during playback of this track.
Dragging the position indicator
While a track is playing you can drag the position indicator to another position to skip or replay certain parts of the track.