If someone calls you mannered, it's really not a compliment. It means that the way you act seems artificial or exaggerated. Chill out.
Manners are the way we behave in society, how we speak and act with other people. Use mannered to say that someone has exaggerated or unnatural manners. In combination with other words — as in well-mannered or mild-mannered — mannered can take a positive meaning, but when it's used by itself, it's a criticism. Your pretentious classmate is mannered, and an actor's stilted, phony performance is also mannered.