This is a free game that is a very simple idea. You pick a champion along with 2-4 other people on your team and then face off against another team of 3 or 5 players (depending on the map). You level up the champion and try to kill other players, with the end goal being to destroy the other team's base. Once the game is over, you repeat.
For you PC fans, this is a great game and a Defense of the Ancients (DotA) clone. Completely free to play and as you level up, you get Mastery points and Rune slots to customize your champions and make them better. Very addicting game for people who like RTS games, but don't want to have to micro-manage a million things because you only control 1 champion.