What does prototype mean here in the jQuery source code?

As an example, copied from jQuery 1.2.6:

jQuery.fn = jQuery.prototype = {
    init: function( selector, context ) {
        // Make sure that a selection was provided
        selector = selector || document;
        ..........
    },
};

I have read some posts here like JavaScript: What are .extend and .prototype used for? and know a prototype can be used in a subclass to extend some methods.

But I cannot understand the usage in the above snippet from jQuery.

Are there any canonical documents describing the prototype?

Thanks.