What does it mean when a girl sticks her tongue out at you?

Depends what way the person sticks it out. If they are doing it with a negative attitude, it means they are bragging, showing they dislike you, or are jealous of you. If they are doing it with a positive attitude, they are just joking with you. If a boy does it to you, he likes you and is teasing you, or he is one of your friends joking with you.

It means "Kiss me quickly before I change my mind."