The behavior score is pretty much what it sounds like — it’s a score for your behavior in the game. It’s a lot like those grades you received for your behavior in preschool or elementary school — N for Naughty, G for Great, or something along those lines. It’s the same concept here.
If you are a mean or toxic player, you’ll have a low behavior score. If you’re a nice and friendly player you’ll have a high behavior score. The max behavior score is 10,000. No one knows precisely how the game calculates this score, but more than likely it has something to do with commends and reports.
Valve uses this behavior score to match players together when they queue up for games. So if you’re determined to be a toxic player have a low behavior score, you’ll be grouped up with similar people.