jQuery("#product_id").val() -- is to get the value of selected option.
The value is concatenated to the "get_detail.cfm?product_id="
When get_detail.cfm is called, it will return a string as a "result" in the jQuery. Then, the result is inserted to the div tag which is "product_detail".
Go to jQuery documentation to learn more about ajax(), val(), and html(). The doc explains very clear and it has some examples how to use them.