The navigation bar is used for navigating through different tabs opened at the same time, such as models, the Explorer or the Administration tab. It allows you to keep several models opened at the same time.
The navigation bar is always visible, regardless of the number of models opened. Navigation is simple: Just click on the model (tab) you want to display.

The position of the tabs in the navigation bar can be changed at any time. Simply drag and drop the tab to the position you want it in. If you want to close any of the tabs, you can do it by clicking on the "x" in the right corner of the tab. The Explorer Tab is the only one that can not be closed, though. This tab always remains open.