If a uri served by joomla is requested for the first time, this plugin will store a copy of the generated html in its cache folder. The next time the uri is requested again it will be simply read from this file, once the initialization of joomla is ready. This extension is sitting on top of joomla's standard caching and I would advise to activate both to gain most performance.
To clean the cache, you can simply use the normal cache cleaner from joomla and erase explicitly the cache from this plugin.