When female cats are ready to mate with a male we call that "heat." You might also call it being "in season." Someone else may have better definition of it, but that's the best I can do for now. If she's in heat she may be getting in a position to accept a male partner and make some kittens.
All cats generally like to have the on their backs right above their tail scratched. Usually they'll raise their hind quarters up in response to being touched there. Even my male cat does it, but only if I touch him.
I don't have much experience with female cats, so anyone with more experience may have a better explanation for you.
Pretty much echo what everyone's already said. If your cat has not been spayed, that behavior would be consistent with going into heat (wanting to make babies). If this is the case, please take your cat to your vet soon so she can be spayed, and meanwhile, please don't let her get out.
If she's already had her spay, then you can put this into the "cute and quirky" category of cat behavior. Both my girl kitties do this when I pet them. Kayla will stick her butt waaaaay up into the air so I can scratch her sweet spot at the top end of her tail. It's normal and lots of cats do this.