It's one of the best indicators of interest I know. It's a subconcious thing women do, which is usually accompanied by other signs as well. The simple fact that she's playing with it, means it's not meant to "straighten" her hair or anything, but something else. It's accompanied by a certain pose women get. They get this girly pose which obviously tells you she's interested.
If she's just playing with her hair out of boredom, it's a bad thing. If she's playing with her hair to get it back into it's normal model, it's a normal thing. If she's playing with her hair playfully (esspecially while talking to you), it's a good sign.
It's not the only thing to look for though.
Full body language is important too. Eyecontact is a good thing. Smiles are a good sign. Nervousness is usually a good sign. Giggliness is a good sign.
You know, women get this look in their eyes when they like a guy that is unmistakably an indicator of interest. It usually doesn't occur when a guy and a girl have eyecontact, it's one of those things a girl does when she looks at a guy knowing he's not looking at her.
If she has her eyes wide open, a hint of a smile on the side of her eyes and her mouth and her mouth hanging open just a little bit, it's almost guaranteed to be an indicator of interest (happens more often than you think).
I'm not going to get into much more detail, it just means sensing another girl's view of another person, nothing more, nothing less. Learn to empathize, learn to recognize emotional indicators in people, it's not that hard, you've just got to open your mind up to it.