What exactly mean?

It's how long the audience is watching your video. It tells you the average and the percent of the views at certain time of the videos.
 
Yeah the audience retention just shows how long each video is being watched, it's helpful since you can see on which parts of your video viewers stop watching, then you know that on that point in the video you might have to rethink a bit and see why people clicked away at that point. Then learn from it and do it better in your next video.
 
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