You want to AJAX load a section of another page on your site onto the current page. Say your eCommerce CMS system creates a dynamic menu of products, but that exists as a subdirectory of your site and you want to use that on the homepage.
The first param is the URL (only works for same-domain requests!) and the second (well, technically it's still part of the first, separated by a space) is a jQuery selector of the part to load. Not passing the second selector param will load the entire page. There is an optional third parameter, a callback function, which will run when the load is complete.