Glicko RD

Can anyone explain what the Glicko RD is?

Rating Deviation.

e.g. it's like saying you're rated 1500... give or take 150.  The larger the number the more of an estimation the system assumes your rating is, and so wins and losses will change your rating a greater amount.  (An RD of 150 doesn't literally mean give or take 150 though).

The more active you are though, the lower your RD will be, the more accurate your rating is assumed to be, and so it changes more slowly.  For example if you stopped playing for 2 months it might shoot up from and RD of 30 to an RD of 200.

Dan, it is the equivalent of the old K-Factor in the Elo rating system.  The higher the RD the greater the rating adjustment for your next win/loss.  The RD increases slowly over time, and decreases when a new rating is given.  This is to simulate something like a confidence in how accurate the current rating likely is - if it came from five recent tournaments, the the RD will be small because the current rating is likely accurate, but if you don't play for the next two years then two years from now the RD will be large reflecting that your real skill may have gone up or down over that time.  The USCF has some PDFs explaining the math behind it.  I think Professor Mark Glickman's site does too, along with (I expect) a good Wikipedia article if you want to dig in a bit more deeply.