In order to make it easier and more precise to rearrange elements along the Timeline, there is a blue bar that acts as a fixed position marker. The Timeline cursor (red bar) features a button with a plus sign (+) that allows you to place the blue marker in the same position the red bar is.
The blue marker gets fixed in the same position even if the red bar moves, and it acts as a magnet for any element that gets close enough to it. That's what makes the blue marker a very useful tool to place elements more precisely.
When you finish using the marker you can hide it by simply clicking on the same button you clicked to place it, only the plus sign (+) will now be a minus sign (-).
You can also show and hide this blue marker by clicking on its specific button in the control bar of the Timeline.