Affordance refers to the function or service that an interface provides.
Perceived affordance is affordance that is visible and comprehensive to the user.
The perceived affordance of the doors is that the one on the left is opened by pulling it. For the right door it is opened by pushing it.
When presenting information to the user we need to use perceived affordance. We need to present content and technology in a way that is familar to the user so that they will instantly know what to do. For example all links within copy need to be underlined. We should never underline anything that is not a link, that will only confuse the user. Users expect a shopping cart to be positioned in the upper right, that is where we should place it. Stick with convensions already established.
Affordance is really about showing the obvious