The prototype of any category is the member or set of members of a category that best represents the category as a whole.
Not everything fits perfectly in a category. Categories are defined by an intersection of properties that make up their members. Members that have all of the properties are the prototype members. Those that contain some, but not all, of the properties are less prototypical.
Prototypical nouns embody extremely and obviously time-stable concepts. ‘Rock’ and ‘tree’ are prototypical nouns.