It comes on every 7,500 miles to tell you to change the oil.
They're still working on a mechanical hand that comes out of the dash and whacks you on the side of the head if you go past 7,500 miles without an oil change. For now, just the light.
I'm guessing you had the "pros" at Jiffy Lube or some such place change the oil last time?
Kudos to you for getting your hands dirty! Wipe 'em clean before opening the manual for the reset procedure. Something to do with the reset button on the dash. I have to look it up every time myself.
Personally, I do it every 5,000 cause I switched to synthetic.