Affordance (Donald Norman, The Design of Everyday Things) is the quality of an object that signals how it should be used. A door handle affords pulling; a flat plate affords pushing. In games, affordances guide player behaviour without tutorial text: a glowing object affords interaction; a narrow ledge affords jumping; a brightly lit doorway affords movement forward. Well-designed affordances allow players to understand a new environment through observation rather than instruction (CRE342 Lectures, see source-cre342-lectures).
Affordances operate alongside other player-guidance techniques (colour, lighting, audio cues) to form the implicit communication layer of level design.
Related: ui-design, player-guidance, level-design, game-feel