It's an algorithm that calculates how accurate your rating is based on the number and frequency of games you play. The lower it is, the more accurate your rating is.
What is a Glicko RD?
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RD partly differentiates Glicko fro Elo.
Glicko is a rating sysem. RD= rating deviation. As a person plays more games the RD lowers under the assumption that the more games played, the more accurate the rating. if a person doesn't play much over a period of time, the RD will increase slowly. So you can look at a person's RD to determine how accurate his rating is. But also, you can look at his RD to determine how many points you'll win/lose by playing him. If he has a high RD (somewhat inaccurate) lose to him will affect you less to compensate for the inaccuracy. If you have a high RD you'll have a lot of rating change after each game, if you have a low RD you'll have slighter changes. Your RD doesn't just lower the more you play, but rather lowers in relationship to the RD of your current opponent. So, everything in sort of interconnected and relatively confusing.
Source:
https://www.chess.com/forum/view/community/glicko-rd#comment-62761
Mine is 28 in rapid and I’m just curious what it means