At this time wax was used to seal letters and important documents, with a personal or identifying symbol or initials (seal) stamped into the wax to signify who had sealed it. This would also provide proof that an important letter was not a forgery.
To open the letter or document the wax would have to be broken, indicating that it had been read. Signet rings were often used to stamp a symbol or person’s initials into the wax seal. Here, Mina uses her wedding ring instead, symbolising the trust she and her husband have built.