It means that your coolant is low. The same thing happened to my car and it turned out the electrical plug at the bottom of the bottle was not working. I guess a bit of rad fluid had leaked onto it and caused a short so the light came on. The bottle is about $120 or something so I just left mine unplugged. You have a temperature gauge and that's all you need. You might find one at a wreckers for a good price.
Don
I filled the tank and even lifted up the sendor float manually and nothing happened. After a few hours of purging the air out of the system it turned off.