Base on what Dan Norman wrote in his book (The Design of Everyday Things) we have two terms that describe objects.
Affordances define what actions are possible (It is a relation between object and a person (user) so for example a chair affords (“is for”) support and, therefore, affords sitting.
On the other hand signifiers specify how people discover those possibilities: signifiers are signs, perceptible signals of what can be done (chair's flat surface and a back support). Therefore signifiers are far more importance to designers than are affordances.
Hope that adds a little bit of a background to the article.