Functions in JavaScript are objects. And like most objects in JavaScript, you can arbitrarily add properties to them. The $ function is just that, a function. So if you want to pop a property onto it and reference a jQuery collection, or reference, you can.
By adding the collection as a property on the $ function, it is one less variable in the current scope. You can examine the keys of the jQuery function before and after if you’d like to see how it affects the function’s topography and (enumerable) property list:
Object.keys($);
// ["fn", "extend", "expando"..."parseHTML", "offset", "noConflict"]
$.root = $("body");
// [<body>]
Object.keys($);
// ["fn", "extend", "expando"..."parseHTML", "offset", "noConflict", "root"]