https://support.chess.com/customer/en/portal/articles/1444777-how-do-ratings-work-
It is the Rating Deviation. It estimates how inaccurate a rating is. The higher it is the more the rating can swing and the less it impacts opponents. With more games, the RD value decreases and if there is a large time gap in playing games it will increase.
What is a glicko rd. And how does it work?