1997 Ford Probe Overdrive...?

Hello, I have a question about a car I want to buy. It is a 1997 For Probe GT. It runs and drives great, has only 124 thousand miles. Everything checks out good on it, but there is one problem that concerns me...the "overdrive" light is flashing. I'm not quite sure what an overdrive is or does, something with the transmission I'm uming...Can anyone tell me why it's flashing??? I really like this car and want to get it