When you activate this nifty wikitool, it empowers your mouse arrow.
When you hover the mouse arrow over a link, a preview of the article or image "pops up", so you don't actually have to go to that page to see it. If the article doesn't preview, you can activate it from the popups menu provided in the popup box (just hover the mouse over the word "popups" and then select "enable previews").
Another popup menu provided is "actions". When you select one, it applies that action to the page specified in the link you are hovering over. Actions include "edit", "diff my edit", "move page", what links here", and many more.
And you can get popups within your popups (by hovering over links in the preview)!
To activate Navigation Popups, click on the gadget tab of Preferences and check the Navigation popups box. In order to activate and use Navigation Popups, you must be logged on.