Whenever a postcode is entered in the Billing Zip/Postal Code field and the Save button is clicked, the validation rule checks the value along with the value of the Billing Country, and, if these are not valid, the account record is prevented from being saved.

You can see how an invalid entry in the Billing Zip/Postal Code field for a GB Billing Country stops the record from being saved with an error message that appears below the field on the edit page in the following screenshot: