window.$ = jQuery = $telerik.$;
To the best of my understanding, there is an Object called $telerik. It has a function $. The function is assigned to the identifiers jQuery and window.$, so we can just use $(args) in our javascripts. Is it correct?
It should mean that we can also use jQuery(args) to find an element.
The $telerik object is given by a set of commercial ASP .NET Web Controls called "RadControls for ASP .NET AJAX".
Recently they have included jQuery into the client-side API, and they decided to wrap the jQuery object inside $telerik for compatibility reasons.
The code you post is only declaring the $ and the jQuery objects globally, so you can reference to jQuery by it, and write code as you normally do.
For more info give a look to this post.