Invented as improved rating system for chess, now used also in multiple computer games and some sports.
Grandmaster 2600 and up
Senior master 2400–2599
Class A 1800–1999
Class B 1600–1799
Class C under 1600
In a roughly abbreviated explanation, players under the elo system are placed at a specifically middle rating, say 1200 in League of Legends. With subsequent games, players can increase their score from victories or decrease their score from losses. The fine tuning of points earned when winning or losing a game are usually made separately for each type of game, as different mechanics are involved.
B: That sucks, there's not much you can do besides get better.