Affordances allow us to take advantage of common experiences and cultural standards to create immediate understanding of our designs. Examples of affordances in physical products include:
A coffee cup with a handle suggests that you should grip the cup by the handle.
Buttons on doorbells are for pushing.
The material and shape of balls suggest they are for throwing and bouncing.
Read the whole thing. If you want to learn more about affordances, Don Norman’s The Design of Everyday Things is a great starting point.