What does Ruth 4:21 mean?

David's genealogy finally reaches Boaz and Ruth's son Obed.

And yet, this verse and the next bring the most confusion to biblical scholars. Matthew's genealogy of Jesus states that Salmon married Rahab (Matthew 1:5). Rahab is Boaz's mother. Considering Salmon's father Nahshon was the chief of the tribe of Judah during the exodus (Numbers 2:3) and the only other women Matthew mentions—Tamar and Ruth—are either foreign or came into the line through dubious circumstances, there's every reason to believe this is the Rahab who hid the spies in Jericho (Joshua 2; 6).

That makes Salmon between twenty and forty years old when the Israelites entered the Promised Land likely around 1406 BC. Salmon has Boaz, Boaz has Obed, Obed has Jesse, and Jesse has David likely sometime after 1040 BC. If Salmon were thirty when he entered the Promised Land and he, Boaz, Obed, and Jesse had their sons when they were 100, this would work.

The Jewish interpretation is more likely. Jews reserved the fifth and seventh positions on a list for honor. In Ruth 4:18–21, the fifth name is Nahshon, the leader of the tribe of Judah during the exodus (Numbers 1:7; 2:3; 7:12). The seventh is Boaz, the hero of the story. In addition, ten generations—like the line from Adam to Noah in Genesis 5, then from Noah's son Shem to Abraham in Genesis 11—are symbolic of a major change in society. Jacob prophesied that Judah, the father of this tribe, would reign over his brothers (Genesis 49:8–10). Most scholars think Samuel wrote the book of Ruth. Scholar Zvi Ron indicates that "The ten-person list is to indicate the transition from the Patriarchs to the legitimate monarchy of David." That is, Samuel is shoring up support for David over Saul who was from the line of Benjamin, not Judah.