What is the learn button. and where is it?

What is the learn button. and where is it?
Right next to the "Any Key" button on the keyboard?? Seriously though, on what devise are you referring to. All i can think about is on a garage door opener to sync it with a cars built in opener. Most of the time they are on the rear of the unit, but sometimes can also be under the light cover. If this is not what you're referring to then i coudnt really say.