If you mean in terms of UR, then just check the rules as Xiander suggested. If you mean where did the term come from:
The Elo rating system is a method for calculating the relative skill levels of players in two-player games such as chess. It is named after its creator Arpad Elo, a Hungarian-born American physics professor.
The Elo system was invented as an improved chess rating system, but today it is also used in many other games. It is also used as a rating system for multiplayer competition in a number of computer games, and has been adapted to team sports including association football, American college football and basketball, and Major League Baseball.
or, if you actually mean Ello, its how they say hello across the pond