What you might be thinking of is one of these:
* Trip Odometer Reset: This button resets the trip odometer, which measures the distance traveled since the last reset. This is often used to track fuel consumption over a specific journey. It's usually located on the instrument cluster.
* Average Fuel Economy Reset: Many Ford Explorers allow you to reset the average fuel economy display. This doesn't affect the fuel system itself, just the displayed average. The method for resetting this varies depending on the year and model of your Explorer. Consult your owner's manual.
To find out how to reset the relevant display on *your* Ford Explorer, check your owner's manual. The location and method of resetting the trip odometer and/or average fuel economy will be clearly explained there.