It's also the light that tells you that you should find someone to connect it to a computer to find out what went wrong. I've had it come on because of a loose sensor but it can also be something far more serious.
If you can't do that, you could do the ecu reset (instructions somewhere on the net) to clear the error and ten see if it comes back. If it does come back then you need to get it to your mechanic.
Good luck!