This is one of the very few things Heroes Of Newerth did correctly before that game died

Essentially a Leaver Rating is this:

If you leave over a certain percentage of your matches before they finish, whether it’s due to quitting the game or disconnecting or going afk, you are no longer allowed to play ranked game modes and then have to play unranked only until that percentage goes back into the green.

In HoN this put a major emphasis on seeing games through to the end because leaving carried a major punishment that wasn’t just a time out.

Edit:

For reference the threshold for HoN’s leaver percentage is 5%. So if you can play 20/21 of your games to completion, you’re fine.

Thank you to u/DeathInNoDisguise for looking it up.