Input handling is how your program listens and responds to what the player is doing — pressing keys, clicking the mouse, moving a joystick. Without input handling your game world exists but nothing in it responds to the player, making it not much of a game.

Every game engine has its own input system and they vary quite a bit in how they are structured. The two most important things to understand with any input system are the difference between held, pressed, and released states. A held input is true every frame the button is down. A pressed input is only true on the single frame the button was first pushed down. A released input is only true on the single frame the button was let go. Getting these wrong is one of the most common causes of input bugs in beginner games — for example using a held check for a jump means holding the jump button will make the player jump repeatedly instead of once.