It represents the rate of the video. It means, when you’ve went to shorten or expand an event, you’ve accidentally (or intentionally) been holding ‘CTRL’ which changes the function of the action. Instead of revealing more or less of the event, it actually expands what is there to produce slower or faster motion.
To correct this, Right Click and go to Properties.
Make sure your Playback Rate is set to 1.000. If it isn’t, make it ‘1’. If it is at 1 already you have nothing to worry about, just ignore the line.
Setting playback rate to 1 will give you the real-time, true rate of what the video is supposed to be. If you set this to 0.5 you would get half speed slow motion and the wavy line you’re seeing would have half the frequency.
Hope this helps :)!
Daniel Hughes
Amateur Writer, Director,
Director of Photography
United Kingdom