Glicko is the rating system chess.com uses, RD = rating deviation.
It pretty much measures how volatile your rating will be. If you don't play for a long time your RD will be higher and if you win/loss you will gain/lose more points.
Here is the article about chess.com's rating system if you are interested:
http://www.chess.com/article/view/chess-ratings---how-they-work
What is Glicko RD? it's in the list thing when you look at your stats.