Most common abbreviation for "Friendly Local Gaming Store", i.e. the place roleplayers usually meet and shop. This is a place gamers like to go to browse new material, chat with other gamers, look for new players to supplement the group and generally keep informed. Lately FLGS have felt the competition from online stores, which can offer lower prices, but not the same atmosphere.
Other variations include the NSFLGS, or "Not-so-friendly local gaming store", usually used, when the shopkeeper is an arse.
Mostly used in sentences like:
Support your FLGS!
or:
Yesterday, I visited my FLGS, and what did I see? The new KoDT issue...!