A reply to someone who doesn't use proper grammar. The words "Learn", "to", and "grammar" are intentionally corrupted for a clever comedic effect.
It can also be used to troll people who are incapable of understanding the joke as they usually point out that "grammar" is spelled wrong.
Yeah, no shit.
Person 1: aint not no place better then usa
Person 2: Lrn2grammer
Person 3: WTF, you spelled "grammar" wrong. Dumbass!
Person 2: trollface.jpg