Typing in extreme shorthand as though you don't have enough time left to actually type out a normal sentence.
Mostly used by people who are either too lazy to bother taking time to write a coherent sentence or people who think it's "cool" because "everyone" does it online.
(P.S. No, we don't.)
Bob: LOLZ @U! U R wya 2 dum 2 uz a comptr!!
Rob: What the hell's wrong with you? You're typing like you're gonna die, stop doing that!
Bob: Wut M I d'ing?
Rob: <signs off>