WordPress Cache Plugin will creates static html files from your dynamic WordPress blog. When a page is rendered, php and mysql are used. Therefore, system needs RAM and CPU. If many visitors come to a site, system uses lots of RAM and CPU so page is rendered so slowly. In this case, you need a cache system not to render page again and again. Cache system generates a static html file and saves. Other users reach to static html page.

Demo: http://www.wpfastestcache.com/