Elo is just a rating point. There are a lot of details, but the main point is that it tries to predict probability of winning. If a player has a rating 200 elo

Elo is just a rating point. There are a lot of details, but the main point is that it tries to predict probability of winning. If a player has a rating 200 elo higher than you, they should win about 75% of the time. When you win, your elo goes up, and the other player's elo goes down. Bigger initial differences result in bigger changes, blahblah, lot of details. it also helps rank the players because not every player just plays every other player, so you can't just have #1, #2, #3, etc. But you might consider the player with the highest elo the de-facto #1 player. But they haven't actually played every other player. So it also only works on a fixed pool or player base. So, you can't directly compare ratings from different organizations, etc. It doesn't actually measure a player's skill, it works closer to a stack ranking of a particular pool of players. A lot of people seem to think it measures "skill" directly in some way, but it doesn't do that in any direct way at all.