The WP Super Cache plugin reduces the load on your server by storing static HTML files for the pages on your website and serving those, instead of running WordPress's PHP scripts every time a page is loaded. Google will penalize your site in the rankings if your pages take too long to load, and using a caching plugin such as WP Super Cache helps speed up the load time. It is especially helpful to use this type of plugin, if you have a lot of other plugins running on your website, because each plugin is another script that must be run each time your site loads and this slows down your load time. You can get the WP Super Cache plugin from http://wordpress.org/extend/plugins/wp-super-cache/. WP Super Cache is installed by default by some hosting companies. Remember to disable WP Super Cache when making design changes to your website, so you can see your design changes in real-time, rather than seeing a cached version of your pages.