Taken from Wiki:

The Elo rating system is a method for calculating the relative skill levels of players in two-player games such as chess and Go.

"Elo" is often written in capital letters (ELO), but it is not an acronym. It is the family name of the system's creator, Arpad Elo (1903–1992), a Hungarian-born American physics professor.

Elo was originally invented as an improved chess rating system although it is used in many games today. It is also used as a rating system for competitive multi-player play in a number of computer games, and has been adapted to team sports including international football, American college football and basketball, and Major League Baseball.

Hope this answers any questions about ELO.