Whenever a country code is entered in the Billing Country field and the Save button is clicked, the validation rule checks the value of the records that are stored in the custom object and looks for the country code.

Using the country code as a match, the IsEmbargo value is returned and evaluated within the validation rule, and, if it is true, the account record is prevented from being saved.

You can see how an entry in the Billing Country, which returns an embargoed country, stops the record from being saved with an error message that appears below the field on the edit page in the following screenshot: