New to PvP, little help

Unknown

Few questions from a newbie after doing some reading & trying to play..

What does MMR stand for?

During the Punisher event I got ploughed because I was constantly being given 40+ people to beat. It took forever to find someone with characters that I'd stand a chance with. So I slowly leveled up Punisher & was able to occasionally punch above my weight. Its really frustrating spending 5 mins to try and find an opponent that I stand a chance with.

With the Wolverine event I've been trying to find a team that I can play against for 45 mins, I'm getting thrown really high level chars. I realise I'm new & my chars are restricting me, but surely I'm not the only new person that doesn't have a string of high level or high HP characters? How can I get teams that are more my level???

I just want to play, not spend 90% time trying to find a team.