The Konami Code is a sequence of buttons normally characterized by a gamepad controller. It was implemented in early Nintendo games, such as Gradius and Contra. In most cases, entering this code (up up down down left right left right B A) would give you more lives, more powerups/weapons, or just reveal some neat secret or easter egg in the game.
Some websites today have also taken to implementing this code, in exchange for showing you something silly. A person on a forum I frequent linked this site a while ago, and it’s pretty neat: