What is O/D button on my 2002 Ford Explorer's gear handle

Does anyone know what is that O/D button on my 2002 Ford Explorer's gear handle!
I've just got the car and i didn't get the chance to ask anyone.

Over drive.

Normally OD means overdrive… Its like an extra gear… But i have not seen an OD button for years… Never knew they still did them

Gear handle? Never mind the OD button it has a gear "handle"!

Overdrive. Allows an extra gear for highway driving, to keep the rpm's down at higher speeds.

The OD button is a lock out for overdrive in other words to keep it from shifting into the overdrive mode at higher speeds. If your driving at moderate speeds and lots of hills and valleys it would be a good idea to press that button to keep the transmission from going into overdrive. It would keep the transmission from a lot of unnecessary shifting and may even extend its life.

It's overdrive lockout button. It just keeps the transmission from shifting into 5th gear (in your case). It does not activate overdrive but rather disables it.

Push it to turn the overdrive off. Use it that way when towing or going up big hills with a load.

It is over drive in and out.
Leave it in
It gives access to a 4 speed box with a 3 speed shifter.

That's the overdrive lockout button. When pressed, it keeps the transmission from shifting into overdrive. This is primarily for towing or heavy hauling applications as it reduces stress on the transmission under those conditions.