Every player carries a rating, updated after every result, oldest match first. Everyone starts at 1500. A win against a stronger player moves a rating more than a win against a weaker one.
How much a result moves a rating
A player's first 20 matches move their rating further, so a new player finds their level quickly. Until then the rating is marked provisional. After that, each result moves it by a smaller, steady step.
Active players
The table ranks players who have played in the last year. Everyone keeps a rating, but a player who has stopped playing drops out of the ranking until they return.
A rating on any date
/v1/ratings/snapshot rebuilds the table as it stood on a date, from matches that started before it and nothing after. That is the table to backtest against: it holds no knowledge of what came next.