A prototype in psychology is the number one, or best, example of a representation for a category in your mind. This helps our brains quickly sort, categorize and understand new information as it relates to previously understood concepts.
As an example, what would be the first thing that popped into your head if I asked you to imagine an idiot? For many of you, it might be someone like Donald Trump or Elon Musk, but when I asked myself that question, all I could remember was the mirror. But regardless, your Donald Trump would be the prototype of an idiot where you compare all other idiots.
That is, Donald Trump would be your reference point for all idiots, and any other idiot would then be compared to your initial prototype idiot as being fatter, smaller, rounder, poorer, human-decency-ruinerer, etc.