The term 'affordance' refers to the actions that a person can take when they look at an object. We all know that a light switch can be flipped leading to the light coming on. But what about on the web?
We all know what a button does in the real world but if we have something like a button on screen, or something that we want someone to know is clickable, I have to put the cues in place to suggest to the user that it can be clicked.
Sometimes though something can be clicked but doesn't give any indication that it can be clicked. Worse still is putting in place a feature that suggest it can be clicked on but in fact doesn't do anything.
Part of knowing what to do is from knowing how the rest of the website works or how other websites work. How the web 'works' has become very deeply ingrained in our perceptual capabilities and forms our schema for using websites. Break this at your peril.
When it comes to website design I sometimes have to advise my client that what they want breaks the rules of 'affordance' or sometimes tries to utilise too many different cues that only then serves to confuse.