Basics:
This is how long people watched your video for, on average. If 100 people watched your video, and 50 watched for 1 minute, and 50 watched for 2 minutes, your average view duration is 1.5 minutes.
Learn:
This is a helpful stat.
It's a good indicator of how good your video is at keeping people's attention.
This is often a good metric of how well you told the story, or how engaging the video is.
Gotchas:
This is again a very misunderstood stat.
Unfortunately, like every stat, just getting a high AVD doesn't mean your video will get tons of views.
It's possible to have a high AVD, but low views, for a couple reasons:
If the thumbnail/title is not good enough to get people to click.
If the video concept is not interesting enough for a wider audience.
Remember that just because "AVD is high" (people who watch are watching the whole video), does not mean "This video is for everybody" (people who don't watch, don't watch because they're not interested).
Advanced:
This stat (usually) goes down a lot when you start going viral.
This isn't a stat to ignore, but it is a piece of the puzzle.
If you're getting consistently low AVD, take a look at your editing style, and see if you can make it more engaging.
Or experiment with different video lengths.