Blender video editor not starting my created video

I have a problem with blender video editor (first time to use & first time in V.E. )
I created my video
press |> to start play animation
two black lines before the start of my video restrict the play
the playing line keep going back and forth between these two black lines.
the plot illustrates more clear my problem.
Small question : how to remove unused lines?

Thanks alot in a head
I hope somebody from experts (in video editing by blender) can kindly help me.