Laughing Out Loud. Hope it helps!

It's an abbreviation of 'laugh out loud'. People are too lazy to write it all out :P

Laugh out loud. It's added at the end of a sentence to mean the person is actually laughing, or that their comment shouldn't be taken seriously. eg. "your an idiot lol" (note the bad grammar which often accompanies such an acronym) It looks rather silly, so you won't miss anything by not using it.