Project Pages are those dealing with the project itself, the wiki internals.
When You have a Wiki about dogs, the Main Pages are about dogs and the Project Pages are about how to manage the Wiki that collects information about dogs. Here it is a little confusing, because this wiki is the Wiki of Wikis. Therefore our Project pages are titled: WikiIndex:Pagetitle and are dealing about wiki maters as well, just like our Main Pages, only more on a meta level.
Our Project Pages should all be found in the Category:WikiIndex