Design AtlasAffordance is a term coined by psychologist James Gibson for "the possibilities for action that an environment offers an actor." Donald Norman brought it into product design, and his most-cited example is the "Norman door" — a door fitted with only a flat plate can only be pushed, so there's no room for confusion; one with a vertical bar handle signals pull through its shape alone. The trouble case is a door that needs pulling but is fitted with a flat plate anyway, so everyone keeps pushing — that's affordance and signifier (the cue telling you what to do) coming apart.
On physical products, affordance runs through touch, weight, and sound as much as sight. Knurling cut into a knob's surface signals "turn me"; a raised lip around a dial's edge signals "hook a finger here and rotate." A concave dish sunk into a button's face signals "press here," because a thumbprint naturally settles into a hollow.
A shadowed rectangle in a UI reading as a "pressable button" is the same physical affordance faked on a screen, and it's why skeuomorphism leaned on it so heavily in early touchscreen interfaces — reusing a shape cue already learned from the physical world, so the interface didn't have to explain itself.
When to use
When checking whether a control's shape (a handle, button, or dial) actually matches how it works.