ROFL is an acronym for Roll on the Floor Laughing. In lower-case it is "rofl".

• Frequently used to describe things that literally are funny enough to make you fall out of your chair, laughing.
• Upper-case and lower-case typically have slightly different meanings. "rofl" is typically a light laugh or chuckle, while "ROFL" is a heartier, louder laugh. Even more extreme is the acronym ROFLMAO.
• When you type only "rofl" in the chat or /rofl in-game, your character will automatically do the /laugh animation & sound. /rofl will appear to other players as "Player rolls on the floor laughing (at X).".

See also Emote.