More specifically, a RS is the side you want to show the world... either it has a pattern you're working that looks nice, is the flat side (knit) of a stockinette stitch, etc. The WS is the side that helps you work the piece but doesn't look as nice as the other. RS means right side and WS means wrong side. I hope that clears it for you!
Yeah, like Suzeeq said... RS means they want that side facing you and the same when they put WS