The meaning of what it means depends on the situation and the way the girl does it. Sometimes, if she is smiling or laughing, she is just playing around, being funny. I like that because it makes me laugh too. If she has this crazy look on her face and she is being tough, gotta get away from her. Turn off.
i have a couple of girl friends who do that to me a lot but its not meant in an offensive way. believe it or not its become a greeting sometimes! other times they'll just call my name then show me the finger. I suppose whether or not you take it as an offense will depend on the girl, how well you know her and the situation.