Repeating Last Action
That key is the F4 key!
In Word and Excel many tasks can be repeated instantly by tapping the F4 key for as many times as you would like to repeat your last action.
Say you would like to highlight something– do it once and then click F4 to repeat that action.
BONUS: This works not only in Word and Excel but in other Office applications as well!
BONUS BONUS: As you all know, I love to save time and keystrokes! You can also achieve this by clicking CTRL + Y!