A prototype is the BEST example or cognitive representation of something within a certain category. Prototypes are used to enhance memory and recall, since you can keep a prototype of something and then match new, similar things to the prototype in order to identify, categorize, or store this new thing. For example, if I ask you to imagine a dog, what do you imagine? You may consider a German Shepard your prototype for a dog by which you compare all other dogs. So if you see another dog, you could say that other dog is small (compared to your prototype), heavy, ugly, beautiful, etc.