The concept is simple. It is to have one standard icon and button to indicate that a page is editable. Much like the orange RSS Feed icon that we have all grown accustomed to, there is now the same sort of icon indicating a page that you can edit (9 times out of 10 a wiki page). Moreover, clicking this "universal edit button" functions just like clicking "edit" on the page. This blog post from SiliconFlorist.com probably sums it up the best, and also see UniversalEditButton.org for more info.
So far more than 20 wikis have jumped on board and you can download the Firefox extension and visit a wiki (like AboutUs) to see what's it all about.