Matchmaking Rating (also known as "MMR") is a number calculated for each player to match them with players of similar skill in PvP.
The intent behind MMR is to match players by skill. This is accomplished by comparing an assigned number (MMR) that adjusts based on the relative MMR of the opponents you win or lose to. Matching players by their effectiveness or performance helps make sure that less experienced players aren't always getting stomped by expert players, and that competitors at all levels are earning their victories by battling players of roughly equivalent ability. MMR isn't intended to be a "PvP score"—its primary function is in matchmaking.