An "action possibility". In other words, something in the design that lets the user know that you can do something.
Once you start to think about affordances, you'll see them everywhere where good design is present. The classic example is with doors. If they have a knob, you know to turn. If they have a plate, you push there. If they have a vertical bar, you expect to pull on it. You can probably figure out how to get through the door without the affordance, but it would be frustrating.
Look at some other objects around your house, especially kitchen, and think about the clues that the designers have given you to know how the objects are to be used.
Edit: I highly recommend the Donald Norman's
The Design of Everyday Things for anyone interested in an introduction to design concepts. Classic book, and a worthwhile read for software developers too.
Once you start to think about affordances, you'll see them everywhere where good design is present. The classic example is with doors. If they have a knob, you know to turn. If they have a plate, you push there. If they have a vertical bar, you expect to pull on it. You can probably figure out how to get through the door without the affordance, but it would be frustrating.
Look at some other objects around your house, especially kitchen, and think about the clues that the designers have given you to know how the objects are to be used.
Edit: I highly recommend the Donald Norman's The Design of Everyday Things for anyone interested in an introduction to design concepts. Classic book, and a worthwhile read for software developers too.