What does Deuteronomy 25:5 mean?

If two brothers live together and one of them dies without having a son, the widow shouldn't marry someone outside the family. Her late husband's brother should marry her and have a child with her. Deuteronomy 25:5 - Modern Text Bible

(This law is about protecting family lines and making sure widows aren't left without support. The brother steps in to carry on the family name.)