Everyone interprets it differently, but in general terms, I would say that it's an extremely rude gesture and you shouldn't do it unless you intend to start a fight or a very intense argument at least. In some instances, people do it as a joke, but only among VERY close friends. Otherwise, it is extremely insulting and you should avoid giving that particular gesture. It's equivalent to saying "fuck you!" or "Go to hell!" if you were to express it in words.