What does it mean if one of your friends calls you ugly behind your back?

It's easier for someone to say things about you to someone else than to say it to your face. Some people actually enjoy talking to people about other people and saying mean things about them. It's called gossip. When the person is pretty sure that what they say will get back to the person, it is a form of bullying. The best thing to do is to recognize that it is a character flaw in the person doing it and do not respond at all.