Shortcuts are key combinations which help you to reach functions immediately. There is no need to find the wished function via mouse in a menu. If you know the shortcut you can reach your goals much faster.

Today’s shortcut: Undo your last command.

Undo last command.
cmd + Z   i.e.   command + Z

This combination allows you to go a step back a correct a mistake you made with a wrong order.

So you can undo deleting a sentence in Word or remove the last applied filter in your favourite picture app. Important: the shortcut is global, but how many steps you can go back is defined by the used program. So feel free to test a bit in your most used app so you are prepared when you need this shortcut.