The light serves as an indicator that your handbrake is on (duh..) and that your fluid is getting low. Sounds simple.
Problem is the fluid in the reservoir will also drop as your pads wear and the calipers drop closer to the disc (nitrodann, feel free to jump in with editorial input if I have the explanation wrong...), so it in fact can be caused by one of four things:
1. your handbrake IS on
2. you have a dodgy light / electrical fault in the wiring
more significantly....
3. your brake fluid IS low (possibly leaking)
4. your pads are worn
Happened to me yesterday at Eastern Creek Nth Circuit, cause pad wear (blame - approach to turns 2 and 17) luckily not sufficient for any ill effects
