Clicking on any area heading in the left hand navigation panel will open it up in your main work area:
You'll notice that this opens up as a tab within the work area. Furthermore, any other areas will open up as additional tabs, and any record opened up from within an area will also open up as a new tab:
These can be closed individually when finished with or, if you have multiple tabs open, you can right click on any tab to close it, close all others apart from that one or close all tabs to the right of it.
Disable tabs is a special option that removes the tabs from the top of the screen. All previously oprn tabs are still available via the tabs button. Note that recently closed tabs can also be accessed via this button.
Tabs can also be re-enabled from this menu.
Open in a new tab is described in more detail here.