The Main Work Area is where MDI object editors are displayed.  Each editor is displayed in it's own tab.

The Enterprise Manager also filters the work area to only show editors that belong to the database that is currently selected/opened.

Tabs may be closed in several ways:

Use the middle mouse button to click on a tab.

Right click on the tab and select from the close options.

Use the close button on the right side of the workspace.  This button is only visible if there are open objects.