LOL stands for Laugh Out Loud. Since you can't hear the poster actually laughing (no matter how loud you have your volume). Many net users will tell you 'hey, this made me laugh. Not just internally, but externally.'

Though sometimes they are full of it. It really didn't make them laugh at all. They just thought it would make you feel better if they put lol in the post. Making you feel better makes them feel better.