What does it mean when the "overdrive off indicator" come on in a 1996 Ford Contour GL 2.0 4cyl?

The other day the "Overdrive OFF Indicator" light came on in my car. I was parked in front of the school waiting for my son to get out. The car had been running for about 30 minutes total when the light came on. I don't know if it makes a difference or not but the air conditioning was on for about 20 minutes. I shut the car off and the light has not came back on since.