What does Ruth 4:7 mean?

The nearest kinsman-redeemer formally withdraws his claim to Naomi's land. He refuses Boaz's challenge to marry Ruth and give Naomi an heir. This could be in part because he does not want the complications which might come from marrying a woman from Moab. His stated reason is that he does not want to impair his own inheritance (Ruth 4:1–6). The man shows his decision by taking a sandal off his foot and giving it to Boaz. This specific tradition seems to have been normal for the culture but is not well documented for us. In fact, the addition of the comment infers the story's audience wasn't familiar with the tradition, either.

Deuteronomy does mention it. It says that if a woman's husband dies without an heir and his brother refuses to marry her and give her a son, "then his brother's wife shall go up to him in the presence of the elders and pull his sandal off his foot and spit in his face. And she shall answer and say, 'So shall it be done to the man who does not build up his brother's house.' And the name of his house shall be called in Israel, 'The house of him who had his sandal pulled off'" (Deuteronomy 25:9–10).

It's possible the woman removes the man's shoe and keeps it as a sign that he has put the responsibility for her well-being on herself. The spit is because it's a disgraceful choice. In this specific case, the man is not Naomi's brother-in-law, and does not carry the same moral or legal obligation. It's likely he didn't incur the same family shame.