The plugins.click_to_play preference blocks plugins from playing automatically but sometimes breaks the page functionality.
The button adds an easy way to toggle the setting on or off and reloads the page.
When you restart firefox the extension sets the preference to true again so you stay protected.
From Firefox 23 on, you have to choose which plugins are controlled by this addon by setting them to "Ask to Activate". (http://kb.mozillazine.org/Issues_related_to_plugins#Click_to_Play)
Due to a major change in Firefox 24, there is no point in using this add-on anymore..