When you do something such as move a mouse or press a button on a joystick you are communicating with the computer by giving it input. If you gave the computer no input then it would have nothing to interact with. This may be as simple as deciding what to do when a player presses a button on a keyboard. The computer has to refer to the actual 1's and 0's within the game program to make these decisions. Output This is the final part of game interactivity.
Output is the reaction that the computer gives to the game player.
This could be any number of things such as moving a character around on the computer screen, playing a sound or animation, etc. With this step the game completes a cycle of interactivity.