The metal horns represent everything metal. It was originally a way to shield against people giving you the mean eye. Ronnie James Dio grew up watching his mom do this on the street when someone looked badly at her. Dio brought the gesture to the world of metal.
The metal horn gesture has always been a way for metal listeners to express themselves about something cool and awesome. The horns represent everything about metal, but when other people use the gesture it becomes "pop-ish", which is very bad!

I saw a behind the scenes of a commercial shoot, where five model sat on a bench, having a break, and when the camera came they all startet throwing horns. Now, unless those model listen to metal, which I doubt, they have not earned the privilege of throwing horns. It would be like if a person saw the Swastica (the Nazi symbol) and thought it was cool. This person the had the symbol tattooed onto his/her body. When this person some day runs into a Nazi, how do you think the Nazi will react? I think he/she will be f-ing pissed. He would probably beat up or maybe even kill the person.
So: Don't use gestures/symbols you have not earned the right to use!!!
Sencerely
The Metalhead of Denmark!