A kinsman is a close relative in the same family. In ancient Israel, the tribe was the largest social and political group. The family was the smallest social unit within the tribe. The relationship between families was carefully organized by rules about who could not marry each other (Leviticus 18).
People who were related, even distantly, received special rights and responsibilities for all family members. It was the right of the kinsman to receive the property of a family member who died without children (Numbers 27:11). The kinsman was also required to buy back property of a relative who had fallen into debt (Leviticus 25:25–28). This was especially important if the relative had become enslaved to someone who was not an Israelite (verses 47–49). When a kinsman (karov) performed this role of buying back property or freeing relatives, he became known as a kinsman-redeemer (go'el).
In the book of Ruth, Boaz is the kinsman-redeemer. “The man is a close relative. He is one of our kinsman-redeemers” (Ruth 2:20). On legal grounds, Boaz had a right to reclaim the property of Naomi. But the law required him to wait for his turn. This was because he was not the nearest relative (4:4). The nearest relative refused to help (verse 6). So Boaz fulfilled his duty as kinsman.