One correction to the above post -
$find is not a jQuery function.
$find() is a function from the Microsoft Ajax Core Javascript library. However, it is deprecated now and replaced by jQuery.
There are few methods like $get, $create, $addHandler etc. which were introduced by ASP.Net Ajax library and these functions provide a shortcut to different methods of some classes managed by ASP.Net Ajax. These can be used directly in javascript if you have included Ajax Script Manager in your ASP.Net web application.
For more details please refer :
http://www.asp.net/ajax/Documentation/Live/overview/[
^]
Now as per your question -
$find('mdlpopup') will find the DOM element having ID 'mdlpopup' in current page.
And, for clearing the confusion with jQuery again-
$(selector).find(target selector); is the syntax for jQuery function but not $find().
I hope this will help you.
Thanks