This refers to the stoning to death of a man or a woman guilty of fornication, who are, or have previously been, married.
Ubaadah ibn as-Saamit reported that the Messenger of Allah (sallallaahu alaihi wa sallam) said, Receive from me, receive from me: Allah has ordained a way for those (women). When an unmarried man commits fornication with an unmarried woman, then one hundred lashes and banishment for one year. As for a married man committing adultery with a married woman, then they shall be lashed one hundred times and stoned to death. Reported by Muslim (Eng. trans. 3/911/no.4191).