A brother's wife, specifically in the context of the levirate marriage obligation. The yebemeth was the woman whose husband had died childless, creating the legal and moral obligation for the surviving brother to marry her and raise up an heir for the deceased.
The yebemeth represents the vulnerable person at the center of God's provision. In a patriarchal society where a childless widow had no security, the levirate law ensured she was not abandoned. God's law repeatedly centers the vulnerable — the widow, the orphan, the foreigner. Ruth as yebemeth becomes the great-grandmother of King David and an ancestor of Christ, showing how God's care for the vulnerable weaves into His grandest plans.