This stands for the Rocket League Championship Series, and this is the professional esports tournament and league for this game. Players from all around the world compete in their various Esports teams to determine who are the best of the best and compete for very lucrative prize pots too. When this event is being broadcast, you can often pop in and watch live through the Rocket League main menu, and even as recently as last year, the RLCS was picked up and broadcast by the BBC, so Guild Esports fans could watch their team live.