It means that someone added a "delete" option in the relationship between these two tables. To prevent it, this option should be removed.
Open Manage | Database | Relationships
Find the relationship line between these two tables and doubleclick it.
Clear the "delete" check box on the Addresses side of the relationship.
Note: Since there can be many table occurrence "boxes" in Manage | Database | Relationships that refer to Invoice and many that refer to Address, there may be more than one relationship where this option may have been specified. You can even set up a chain of "cascading deletes" where deleting a record in table A deletes the related records in Table B which deletes the related records for each of the deleted B records from Table c...