How does ranked worked?

LP is League Points. Points range from 0-100.

On the ranked ladder you are placed in a Tier (this is a metal Bronze, Silver, Gold ect.) Within the tier you are placed in a division (I, II, III etc.) Low number being Better.

So your league for example could be Silver II and you could be at say 90 LP. When you reach 100LP (earned by winning) you will play a promotion series to advance to a higher League.

MMR = Match Making Rating

This number is kind of a hidden stat, and works behind the scenes, this determines who you get matched against and with. ELO is basically the same thing. Its an older Term that was used before league had "Divisions" for ranked play, back before season 2.

When you start playing Ranked, you play 10 matches to determine your initial ranked placement. Its easy to keep track of your first 10 matches because you can see how many you won before it places you into a League. (this system is changing next season)

Next season it will show your placement from the 1st game you play, instead of having to turn out 10 matches first.

Edit : you can play ranked right now to help raise your "MMR" this will in turn give you a better chance at placing into a higher League once the season actually starts.