Title.
It's basically how much rating you win/lose after a game. sometimes you can just not gain any rating at all.
Rating deviation on one Variant, maybe Blitz, Rapid, or Classical, is basically effected by the number of games you have played on that Variant and can be change the number of rating point to gradually increase less.
It's a measure of how unreliable your rating is, based on how many rated games you've played in the category (Blitz, Rapid, etc) and how recent those games were.
The deviation goes up if you're not playing rated games and comes back down when you play again.
And, as others state, high deviation (=low reliability) means that your rating will change more when you play, and also that your opponents' ratings will change less.
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