Display, which is the output of a system, is everything in the game that can be perceived by the player. Another way to look at it is as feedback to the player.
Display can (and should) involve many senses, not just sight.
(For example, in a fighting game, if you press the button for a punch, what kind of feedback could you expect? Punching animation on player character, punching sound effect, player character power level dropping, enemy character reaction [block, hit, dodge, etc], enemy character reaction sound effect, enemy character HP bar dropping...)