From Teflpedia

A prototype is an idealised representation of a member of a conceptual category. Such a representation is said within prototype theory, to be prototypical.

For example, consider a prototype of the category "animal". Consider a "typical animal". The likelihood is that you thought of a non-specific medium-sized mammal, with four legs, a head, tail, and fur. While this description fits some animals, such as a dog, fox or sheep, others are bigger (e.g. elephant) or smaller (e.g. rat) but have the same basic body plan. Further away in the category are birds, reptiles, fish, invertebrates, etc, scientifically classed as animals but conceptually further away from the prototype.