Basically, it lowers the delay that Safari has before it starts displaying a page. The default is 1 second -- that means that Safari won't display anything to you for one second, and then it'll display the page. If the page is done loading in under one second, you still wait the entire second to view the page. Lowering this value makes Safari start showing you the page sooner, but at the risk of displaying an incomplete page, and things may "jump around" the page a little as the rest of the page loads.