Seals are used on documents to prove that they really did come from the person who signed it (they “authenticate” the document). A seal could be put on the letter itself, or on the envelope where it is stuck down. The writer would pour some wax over the joint of the letter, then press a ring or metal stamp (called a matrix) which has his official mark on. This meant that no one would be able to open the letter and then close it again, because the seal will break when the letter is opened. Most governments still put seals on important documents. Letters do not normally have seals any more, even important letters.
Sometimes lead, pewter or ivory were used instead of wax.
Each seal is different from any other. This means that an important person (e.g. a king) has his own personalized stamp.