What is the green indicator light on my dashboard that goes on and off while I'm driving.

It essentially means that you're driving the car conservatively enough to maximize fuel consumption. If you floor the car (or basically accelerate it with a little vigor) the eco light disappears momentarily until you ease off the gas and resume cruising speed. Depending on the type of vehicle, in the ECO mode, the throttle responsiveness, transmission gear shifting, or the electric systems will be tuned to reduce their energy draw. You can refer to the owner’s manual for specific information on what your ECO light means.