ELO is a rating system used to rank players in competitive sports, video games, and other activities. The reason it's called 'ELO' is because it was created by Arpad Elo, a Hungarian physicist.
Basically, it works by giving each player a score based on how well they do in each game. When a player does well, their score goes up. When they do poorly, their score goes down. The better a player's score is, the higher they will be ranked in comparison to other players. When two players compete against each other, the one with the higher score will usually win.