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You're only going to see that when you're zoomed into the timeline really close. It's showing you the duration of the frame you're looking at.
I am unable to understand what mean.
Also I do not mark it as correct answer. I do not know who mark it.
Same issue again
As noted ... it's very simple.
You are "zoomed in" so close on the timeline that each fram actually covers a bit of horizontal space in the timeline. So that horizontal blue bar shows the entire time the current frame will be visible in the monitor.
Normally, when you are not zoomed so far in, the frames take so little horizontal space on the timeline the line is all that is needed.