MMR is your Matchmaking Rating. It is a number value between 0-2500+ that Riot gives to players to determine how strong they are when compared to others. Your MMR in League of Legends takes into account your performance and adjusts itself to keep a fair ranking of you against other players.

How can I fix MMR?

0:094:385 TIPS & TRICKS TO FIX MMR + LP GAINS!| League of LegendsYouTube

Does unranked have MMR?

Unranked is a multiplayer playlist, available for all players with a Clearance Level above 10. Unranked shares the same hidden MMR, which is used for Quick Match matchmaking as well and pairs players based on the same system.