it's a theft deterrent that goes out when the key is inserted and turned to accessory, other than that it has no function and isn't connected to anything else. I think some of them with airconditioning do have a second light, but not sure on that.
There is a blinking light on the ECU itself, and the check engine light can also rapidly flash which is to do with the alarm/immobiliser. When the key is at accessory the ECU light and check engine light both come on solidly lit, when you start the car they go out, or flash rapidly if there is an issue with the key.