The Elo rating system, named after physicist Arpad Elo, is how Chess.com, Lichess, and FIDE all estimate a player's strength. The core idea is simple: your rating is a prediction. If two 1200-rated players play, the system expects each to score 50% over time. Beat a player rated 200 points above you and you gain more rating than beating someone 200 points below you, because the system was more "surprised" by that result.
Expected score is the probability the formula assigns to you winning, based purely on the rating gap — it's a number between 0 and 1, shown here as a percentage. K-factor controls how much a single game can move your rating: Chess.com and Lichess use a higher K-factor (bigger swings) for new or provisional accounts and a lower one once your rating stabilizes, which is why a beginner's rating moves fast and a titled player's barely moves at all.
Use this to sanity-check a rating change you saw after a game, or to see how much a single result against a much stronger or weaker opponent would actually move your number before you play it.