It's one of the handiest "tricks" for boosting your Drupal site's performance! As you move your JavaScript and CSS files to the footer, you'll instantly lift some of the “pressure” off your web pages, which otherwise would strive to simultaneously render both the HTML and the JavaScript (and the CSS, of course). In other words: you'll then be better organizing the whole page rendering process, ensuring that the “bulk” content gets served to your users first and, immediately after, the loads of JavaScript and CSS files. Now allow me to point out the options available to you for making this move: