Match Making Ratio. It's a hidden skill rating giving to you by Blizzard's complex rating system. You are never allowed to see it, but it determines "Favored, Teams Even, and Unfavored". It also determines league.
My MMR must be huge, I went from Bronze to Diamond in 3 game
My MMR is bigger than my dick, and my dick's bigger than Mount Everest. 2+2=a big dick.