Obscene describes something that is morally offensive in a sexual way. It's never a good idea to use obscene language at school.
Certain films, books, magazines, and other forms of entertainment are considered obscene because they portray sex in a very frank way that some people find vulgar and lewd. Obscene material, language, and jokes are considered taboo in polite society. Obscene can also be used to describe something repulsive — like when your friend ate an obscene number of snails at the Bastille Day bash.