Affordance is a design property of a physical thing that suggests how it should be used. A ball affords rolling. A door handle affords pulling.
Whilst you can roll a cube, the physical properties of a cube less afford rolling than a ball. A ball better affords rolling than a cube.
You can push a door handle, but a door handle better affords pulling than pushing.
Take a look at this photo I took of a contactless style of payment terminal at a local grocery store. This is the kind of terminal that, with a compatible credit/debit card you can just tap, hover, or wave you card in front of the machine rather than inserting and keying in a pin number.
Notice the extra handwritten instruction taped to it: “HOLD CARD BELOW”.
The fact that the staff of the store had to modify the machine in this way gives us a clue that there may be a problem with the design. It suggests that some customers were holding their cards in the wrong place, perhaps at the top of the machine, and thus not initiating a payment.
We can see that the only perceived affordance of where to hold your card is the little diagram in the middle.
Whilst I’m not a hardware designer (and without perhaps performing some usability studies) I thought it would still be fun to mock-up some changes.