Documents and other items can be secured by a seal. A small pool of soft wax is run over the join and before it hardens this is impressed with a metal design, which should be peculiar to the sealer (signet rings were originally meant for this purpose). When the seal is hard, the document cannot be opened without breaking it. Deeds which are sealed are effective even though no consideration has been given. Various other legal documents, for instance conveyances, also require seals. Sometimes a thin wafer of wax is still used, but today it is more common to find that documents are imply impressed with the mark of the sealer.
Reference: The Penguin Business Dictionary, 3rd edt.