What is MMR? How can I use it?

I’ve seen a couple people talking about this, what’s it?
And please explain what it does, not just the name…

Matchmaking Rating.
It’s pretty self explanatory… right?

Win a bunch of matches and it goes up. Lose and it goes down.
Doesn’t really work very well in it’s current state.

MMR? Jeez what’s with the acronyms?

To expand upon twysta, it determines against whom you are paired. So say you have won 10 battles in PvP and lost none since the start of your game. You will be pared with people that have also won 10 battles, or close to it. If you win an abnormal amount of battles in a certain tournament, say if you’re going for a certain ranking, your MMR will go up. This means you will be paired with people who have better teams than yours, and you must lose for it to go back down.
Hope this helps!

PvP? Jeez what’s with the acronyms?