The timeline displays in time or beats mode. It also contains markers which includes tempo changes, key changes and time signature changes.
The playback indicator shows the current playback time or where playback will occur when you click "Play" on the Transport. It is made up of a green arrow on the time line and a line.
If you click on the timeline it will set the playback indicator's position and cause playback to restart from that position.
Time vs Beats Mode
Time mode sets all controls and the timeline into minutes:seconds:milliseconds.
Beats mode sets all controls and the timeline into measures:beats:fractional-beats.