A prototype may have an implicit reference to it's parent prototype. This provides inheritance through a prototype chain that is analogous to the super-class and sub-class mechanisms in a class-based object-oriented language.
This allows for properties to be overridden or provided by parent prototypes if the properties are not implemented in the target object.
In a class-based object-oriented environment, as a general rule, state values are embodied in object instances of a class but methods are contained in the classes themselves so the inheritance only projects structure and behavior down through the sub-classing mechanism.
In the prototype-based inheritance the state AND methods are carried by the objects so structure, state and behavior are all inherited down the prototype chain.
All objects that do not contain a particular property and that are descended via the inheritance tree from a single object that does contain that property will all share that one single property instance.