The particular duty of the kinsman that is highlighted in the Book of Ruth dealt with property. There was a threefold duty of the kinsman—to respect the blood of his relative, the liberty of his relative, and the property of his relative. If for some reason the property or the land was forfeited by the man who owned it and was lost to him because he was poor, it was the duty of the kinsman to try and redeem it until the year of jubilee.
In the Mosaic law, God made a provision for his people to experience something of a typical redemption, through a close relative, from the burden of sin and misery. The story of Ruth is the well-known story of redemption by a kinsman redeemer. In his excellent commentary on Ruth, Iain D. Campbell explains how there were three circumstances during the old covenant era in which God gave provision for there to be a kinsman redeemer.
1. Blood—the kinsman had the duty to respect the blood of his relative.
There were some circumstances that could arise in the course of family life in the Old Testament that required the services of a kinsman. If, for example, the blood of a member of the family was shed—if someone was murdered in the family, it was the responsibility of the kinsman to avenge the blood of his relative.