Glicko is a rating system invented by a math professor that attempts to find your true rating quickly by making large swings in the beginning that become smaller as time goes by and you've played more games. The Rating Deviation gets smaller unless you stop playing in which case it begins to rise again. I'm sure you can find a better explanation than that though. Google it and you can even find the formula. Very popular topic in the forums. Check some of these threads out:
http://www.chess.com/forum/search?keyword=glicko+RD
what does glicko Rd means in the states?!