A rating is an estimate, and some estimates are better than others. A rating built on five games says much less than one built on five hundred, and a rating you earned a year ago may no longer describe how you play. Ratings on Maia Chess now use Glicko-2, a rating system that shows these differences (and the rating system used by many others).
Provisional ratings
A rating is provisional while you are new to a mode, or when you have been away from it for months. A provisional rating is followed by a "?", as in "1650?", wherever your rating is shown: after games against Maia, in Hand and Brain, in puzzles and in Bot or Not.
Playing more is all it takes to remove it. Once there are enough recent results, the rating becomes established, and thus eligible for the leaderboard.
Your profile
Each rating on a profile now says whether it is provisional or established, and where it stands on that mode's leaderboard: its place, or the reason it isn't listed.
The leaderboard
The leaderboards list established ratings of players who have been active recently. If you aren't listed but your rating seems high enough to qualify, it is because your rating isn't established. Play more games to earn an established rating.
Also new on profiles
Profiles show when a player joined and when they last played, and game lists say when each game was played.
For everything else that changed, see What's New on Maia Chess.