What does it mean when a girl says they will do anything for you?

Just tell her (: She probably won't believe you on the spot, but what you do is sort of prove it over time. You're actions will have to line up with what you tell her, hence the saying, "Actions speak louder than Words." If it's just a phase and you don't mean it, then don't tell her at all. It hurts a lot when a guy lies to a girl.