Parameters class="modal" tells Joomla to open the link in a modal window href="/contact.html" is the URL of the contact form tmpl=component tells Joomla not to load the template. The modal windows works even without this parameter, but the template graphic, the website menu and all the rest would be loaded within the popup window.
Note that if you already have other parameters, you need to use the & instead of the ? at the beginning of tmpl=component, like in the following example: http://website.com/index.php?option=com_foxcontact&view=foxcontact&Itemid=117&tmpl=component x:800,y:500 are the modal window width and height. As far as I know, this means that the modal window has a fixed size. As a consequence it is not responsive.
As an example, take a look to a live demo embedded in a modal window: