The light comes on every 5000 miles from last reset. It's to remind to your perform required maintenance, which is at minimum, an oil change. You should reset the 5K mile count once the oil change is performed, if the place doesn't do it for you. Toyota dealers know how to do it (it's in the manual and requires no special tools) and have always done it for me when I go there for an oil change. Others might not and you might have to do it yourself.
Unless you had a newer Prius, then the 5k reminder could mean only Rotate Tires since oil changes are at 10k intervals using synthetic oil.
