Open editors and views can be either active or inactive. The active editor or view is the one whose editor tab is the same color as the window title bar. The workbench menubar and toolbar contain operations that are applicable to the active editor.
Any number of editors can be open at once, editor can be active at any one time. The same is true for views. The choice of active editor determines the menu and toolbar contributions.
When multiple editors are open, they will appear stacked in the editor area. You can change which editor is active by clicking on the editor's tab. The name of the file being edited appears on the editor's tab.