The ECU knows how many miles have elapsed since the last time the service light was reset.
It lights up once the recommended service interval mileage has passed (on the Xciting 250, for example, it is 1200 miles).
Once the warning has been reset, it starts over at 0.
If the light is on, you should change the oil. It doesn't have to be a drop-everything-now-and-do-it type of thing, it can wait a week or two.
Once the oil is changed, reset the lamp. You should also check your user manual to see when the transmission oil needs to be changed; they typically are changed at the same time, but there is no light for this.
This way you never destroy your drive train because you forgot to change the oil.
