elo is a rating system which chess often uses. Arpad Elo is its inventor and where the system name comes from. It should be noted that other rating systems also exist with subtle differences (such as the k-factor variable in determining rating changes for less active accounts) as chess.com technically uses glicko rating system and not elo. The difference is negligible, so if you say elo chess.com rating, people still understand what you mean to say (even though technically incorrect).
Although some math nuances may vary, the concept of any rating system is the same. It is a measure of a player's estimated chess level based on past performance. Ratings are an estimate and always will be. It is NOT the same as chess knowledge or chess ability even though higher rated players tend to be more knowledgeable and of greater chess ability on average.
