It means she's confident and feeling social. Anyway, that's what Baby's tail means when she holds it like that.
Put your hand out and see if she headbutts it.
(Jerky, twitching tail movements mean back off; the cat wants to be left alone. But long, exaggerated sweeps mean it wans to play and/or pounce on something.)
I had one really shy shelter kitty, who, when she realized that I wasn't going to hurt her and that petting felt good, would swish her upright tail back and forth in a rhythmic motion like a windshield wiper. flip-flip...... flip-flip.....