SWAG is an acronym meaning "Stuff We All Get." The term was created by convention regulars to describe all the stuff they get whenever they go to conventions. There's good SWAG (cool freebies, drinks, etc.) and bad SWAG (flyers, useless information printouts, ads, and business cards).
SWAG is not to be confused with "swag" which is abbreviated from "swagger". "swag" in this context is used to describe a style, a strut, a brag, a boast, or a vaunt.
Geek 1: I just got back from E3!
Geek 2: Cool! So did you get anything?
Geek 1: Mostly SWAG.
Fan 1: I'm going to a large Star Wars convention this weekend. Want to go?
Fan 2: Sure. Is there going to be SWAG?
Fan 3: Definitely!