What is this timestamp and functions?

The light gray bar shows the duration of the motion blur in the Timeline when Motion Blur is turned on or the duration of a frame if Motion Blur is not turned on. It disappears when you zoom out on the Timeline. 

I can't see the frame numbers, so I can't tell you if Motion Blur is turned on, but in this Timeline, the motion blur is on, and the shutter angle and shutter phase are at the default values of 180º and -90º. This simulates the motion blur you get with a standard motion picture camera.

I suspect that your comp does not have Motion Blur turned on in the Timeline, so you are seeing the duration of the current frame. If motion blur is turned on, then the Shutter Offset is 0º.