See how matches affect your rating in the league. Simply enter the ratings, then pick the result of the match to see how ratings change. Newer players' ratings move faster, so it also needs to know roughly how much each of you has played.
Why does it ask how many matches you've played?
Early on, the league is still working out how good you are, so every result moves your rating a long way. Once you're around ten matches in it has a firm idea of your level, and your rating settles down and moves in smaller steps. That's why two players can gain or lose very different amounts from the same win, if one of them is brand new and the other has played all season. Below three matches your rating isn't published yet, since the system is still calibrating.
You can win and still lose points
If you were the clear favorite and the match was closer than expected, you can come out slightly down even though you won. Beating someone well below you is expected, so it earns very little, and scraping past them says you played below your level.
You can lose and still gain points
Taking games off someone much stronger shows you're better than we had you down for. And early in your career, simply playing matches sharpens the system's estimate of you, which lifts your rating on its own.