This shortcut will repeat the last action again, when available. For example if you just applied a border and red fill to one cell, you can use this shortcut to apply the same formatting to one or more selected cells.
On Windows, you can also use Control + Y for this shortcut. You can add the "Repeat" command to the Quick Access Toolbar to see when an action can be repeated — when it's possible to repeat an action, the command will highlight and become available.
On a Mac, the number of actions that can be repeated is more limited.