If a boy says you're a slut what does that mean?

It means you dress with very little clothing, on purpose to draw attention to yourself. And when other men go for you, you flirt and tease, regardless of whether or not you have a boyfriend thereby completely ignoring his feelings, and leaving him out to rot.

If you don't have a boyfriend, and someone calls you a slut, it means you hook up with too many people, and party too hard. It means you're easy to get in bed with really, and that is really unattractive for men who are looking for women for long term relationships.