Combination of FOCL (Falling off chair laughing), ROFL (Rolling on the floor laughing) and LMAO Laughing my ass off). FOCROFLMAO therefore is "Falls off chair rolling on the floor laughing my ass off".
As a reaction to a funny comment or joke told over the internet.
E.g. Joke Teller: ...so the guy replies, I didn't know your dad was a pharmacist.