An “affordance” is a perceived signal or clue that an object may be used to perform a particular action. A chair sits at around knee height and appears to provide support. It affords sitting. A toothbrush has a handle a little longer than the human palm. It affords gripping.
All of the objects that surround us have affordances: some are explicit (the “Push” sign above a door handle), and others are hidden (a chair could be used to break a window or used as a weapon). The term was first coined by psychologist James G. Gibson, then introduced to human-computer interaction by Donald Norman in his book The Psychology of Everyday Things, required reading for budding industrial and product designers everywhere.