Numbers 30 turns from public offerings to personal promises. Moses spoke to the heads of the tribes: When a man vows a vow to Yahweh, or swears an oath to bind his soul with a bond, he shall not break his word. He shall do according to all that proceeds out of his mouth. A vow was voluntary, but once made, it bound the person before God. Ecclesiastes 5:4-5 later says it is better not to vow than to vow and not pay.
The chapter then addresses vows within households. A young woman in her father's house, or a wife, could make a vow, but the father or husband could make it void on the day he heard it. If he said nothing that day, it stood. If he canceled it after letting it stand, he would bear her iniquity. In that society, the head of a household carried responsibility for its property and obligations, and the law protected the family from rash promises.
The vow of a widow, or of her who is divorced, would stand against her, because she answered for her own household. The point throughout is that words spoken before God are serious. Jesus raised the standard in Matthew 5:33-37. Instead of relying on oaths, His followers should be so truthful that a simple yes or no can be trusted. God Himself is the model, for every promise of God is Yes in Christ (2 Corinthians 1:20).