popup is an integration of overLib, a library used for popup windows. These are used for context sensitive information, such as help windows or tooltips. popup_init must be called once at the top of any page you plan on using the popup function. overLib was written by Erik Bosrup, and the homepage is located at http://www.bosrup.com/web/overlib/.
As of Smarty version 2.1.2, overLib does NOT come with the release. Download overLib, place the overlib.js file under your document root and supply the relative path to this file as the "src" parameter to popup_init.