Sometimes when you’re working at a very large magnification, it can be hard to see the overall picture. Navigator panel saves the hassle of zooming in an out constantly. Currently active area can also be changed (shown as red rectangle) by clicking on within the panel. The colored box in the Navigator (called the proxy view area) corresponds to the currently viewable area in the illustration window.
To open up the panel go to, Windows -> Navigator. Below figure shows the artwork with its thumbnail view in Navigator panel.