Undoing and Redoing
Before I got my Apple keyboard, I had a third-party model that was cheaply made. The paint started to wear off the letters. As a writer and an artist who works mainly in Photoshop, I use the keyboard a lot, particularly the Cmd+Z combination. That's why the Z was one of the first keys to fade to black.
Mac users know that Cmd+Z means Undo. Windows folk know this combination as Ctrl+Z. Same deal. It undoes the last thing you did. To be able to undo with just a keystroke is a wonderful thing. It gives you the freedom to make mistakes, to experiment; and that is how you learn. (I often wish life came with an Undo key.) If the Shortcuts bar is displayed, you can also click the Step Backward button to undo the last action. To redo any change you've undone, click Step Forward or press Cmd+Y (Mac)/Ctrl+Y (Windows). To undo multiple changes, click Step Backward as many times as needed, or press Cmd+Z (Mac)/Ctrl+Z (Windows) multiple times.
You can also find Undo at the top of the Edit menu, but the keystroke combination or the toolbar buttons are really easier to use.