A seal was used to stamp the wax sealing important documents before the days of glue and Sellotape. It usually bore the coat of arms or mark of its owner and was a security device to ensure that the papers weren't opened before they reached their recipient.
Wold is John Durbeyfield's way of saying "old". Throughout the book, Hardy presents the speech of rustic characters phonetically rather than in standard English. If you have trouble making sense of any of it, try reading it out loud.