Working with Toolbars
Toolbars hold buttons that access shortcuts to commonly used commands. The face of each button contains a picture called an icon that indicates the button's function. If you're not sure what a button does, position the mouse pointer in the button and hold it still for a moment. A ScreenTip appears, displaying the function of the button.
If a button on a toolbar appears dim, the feature is not currently available to you. Clicking a dimmed-out button does not result in any action. For example, the Redo button on the Standard toolbar is dimmed unless you've clicked Undo to reverse something you've done in the program.
Excel is ...