Very little has been said about Explore mode in the publicity for Sitecore 8. This is probably because of its previous life as a marketplace module where it was known as the Sitecore Experience Explorer. That's a real shame, because most developers and users won't have been aware of this feature. I think It has the potential to be a truly powerful tool that most digital marketers would find incredibly valuable. It might even be a great learning tool for marketers having difficulty grasping the some of the core concepts of Sitecore.
You can access Explore Mode from the Experience Editor ribbon by choosing Other > Explore. It's split in to two sections, "Settings" and "Viewer", which are represented by panels at either side of the screen.
Explore Mode Control Panels
The Settings panel is where you add in the visitor data you want to simulate. You can add this manually or use presets.
The Viewer panel shows data resulting from your simulation - goals triggered, current campaigns etc.
Experience Mode is split in to 2 sub-modes, "Journey" and "Fixed", which specify fundamentally how you want the tool to work.
Journey mode behaves exactly as if you were a normal visitor to the site, accumulating profile scores, goals, engagement value etc. as you land on each page.
Fixed mode keeps all the specified settings in place as you travel around the site, allowing you to view any page with any configuration you want.
Despite the lack of fanfare, Sitecore have done a good job of documenting Explore Mode. The following articles are currently available on the documentation website: