What exatly does a friend code do and how do you get them?

I see friend codes on all my games but I have no idea what they are or what they or how to get them to appear

Allows you you play against/along with other people via wifi

They vary from game to game. For example...your friend code on say..Mario Kart...wont be the same as your friend code on Animal crossing. Each game has a section to go to, to get your friend code. Depending on which game your playing- it'll be in a different spot. In animal crossing..you have to go to the gate keeper. In Pokemon you get your code from the receptionist at the desk.