What is the meaning of the line displayed at the top of the timeline?
The gray (grey) lines at 1 are the In and Out points common to video and audio.You can add/delete from the [Mark] menu.
The blue lines in 2 are the video-only In and Out points, and the green are the audio-only In and Out points.
You can add/delete from the [Mark] menu.
3. indicates the render cache creation status. Blue is already created, red is in the process of being created or waiting to be created.
In and out points can also be added from the pop-up menu displayed by moving the playhead to the desired position and right-clicking the memory area.