Good designs are intuitive. Affordance is the capacity of a tool to indicate, by its look and shape, how it is to be used.
The concept of affordances originates from ecological psychology; it was proposed by James Gibson (1977, 1979) to denote action possibilities provided to the actor by the environment. In the late 1980s Norman (1988) suggested that affordances be taken advantage
of in design. The suggestion strongly resonated with designers’ concern about making possible uses of their products immediately obvious, and soon the concept came to play a central role in interaction design and Human-Computer Interaction (HCI).
According to Don Norman ” Plates are for pushing. Knobs are for turning. Slots are for inserting things into. Balls are for throwing or bouncing. When affordances are taken advantage of, the user knows what to do just by looking: no picture, label, or instruction needed.” (Norman 1988, p.9).
According to James J. Gibson’s “The Ecological Approach to Visual Perception.” Affordance is about a perceiving animal and its environment. As the animal perceives its environment, it distinguishes possibilities for action in the environment. An open terrain affords walking. A cliff affords falling to your death. A rock on the ground affords tripping while walking, and it also affords throwing. So an “affordance” is some possibility for action that is mutually specified by the agent and the environment.
Essentially, that is it. If you’d like, there is much more to be said about Affordance on the Interaction Design Foundation website.
Source: Affordances and Design, Victor Kaptelinin, Interaction Design Foundation.