yeah as AR6Rider said, it's how you tag things on twitter.
so for example somebody says, "my daddy bought me a BMW and not the Porsche i wanted #fml"
you can click on the "#fml" and see tweets from other people on twitter that contain that same stupid tag.
it is extremely annoying and stupid in my opinion. even when used ON twitter it's idiotic, but when used on facebook or in text message or any other place, it makes me want to rip that person's head off and shit down their neck