The playhead is a vertical line with two arrows at each end that indicates the current position of playback.
Pressing P will set the playhead to the current position of the pointer, if it is within the editor track area.
A Left click anywhere on the timeline (rulers) will move the playhead to that position.
Click on either the primary or secondary transport clock and edit their value to move the playhead to a specific position.
Click Right on the marker and select either Locate to here or Play from here.
Alternatively, place the pointer on the marker and press P to set the playhead.
When the loop transport button is pressed, the playhead will jump the start of the loop range, and continue to the end of that range before returning to the start and repeating. While looping, a light green area is displayed in the time ruler over the tracks to show the loop range.
By default, looping is bound to the l key.
For more information on defining and altering the loop range see Loop Range Markers.