Initially, your question perplexed me. After delving into this beautiful love story I was still none the wiser. Why uncover his feet? Was there some significance about this single act? Four hours later, after immersing myself in this story, I gave up. There was no obvious explanation to be found in the Bible. However, I think I should share with you what I found, and then my conclusion at the end...
Boaz is a middle-aged, well-to-do- Israelite. Ruth and her mother-in-law Naomi are both widows and must fend for themselves because they have no Jewish husbands or sons to take care of them. Ruth first encounters Boaz when out in the fields, gleaning. Although Ruth is from Moab, Naomi is a Jewess and she knows about the Leverite law on the obligations of kinsmen (Genesis 38:13-30).
On the basis of Hebrew law, Ruth was entitled to call upon her nearest of kin to fulfil the various duties of a responsible kinsman. Since Boaz is kin to Naomi and her family, she sends Ruth to the place where the menfolk thresh the harvested grain and tells her what she must do to secure protection for them both.
Naomi tells Ruth to wait till Boaz falls asleep, then uncover his feet and remain there. Later in the night, Boaz awakes and asks Ruth who she is. After Boaz realises that Ruth, a widow, is kin to him (through Jewish marriage), and seeks his protection, he blesses her and promises to take care of her situation. Her reputation as a chaste and honourable woman has gone before her.
Ruth then asks Boaz to cover her with the corner of his garment. There was nothing improper going on, although it would be highly irregular for a widow to be found sleeping at the feet of a man to whom she was not married. Boaz does not hesitate to cover Ruth with his garment and to allow her to remain at his feet. First thing in the morning Boaz will speak to a closer relative, but if he does not want to redeem her, then Boaz will. That means marrying her.
By turning up at night to uncover the feet of Boaz, and laying down by his feet, Ruth's actions amounted to a marriage proposal under the well-known provision of the Hebrew law of the kinsman redeemer (Leviticus 25:25).
Why did Ruth uncover Boaz’ feet? So he would wake up during the night because his exposed feet would become cold as the temperature dropped. He would wake up gently and naturally, without alarm. A simple, practical way to ensure Boaz would not sleep till dawn.