Playback Indicator
The playback indicator shows the current playback time or where playback will occur when you click Play on the Master Bar.  It is made up of a green arrow on the time line and a line in the Sound Workspace

If you click in the Sound Workspace it will set the playback indicator's position and cause playback to restart from that position.  However, you can configure the preferences to not change the playback position and playback indicator when click in the Sound Workspace.  Otherwise, click the time line to set the playback position and indicator.