A vow to God means something like saying, "If God does 'this', then I will give 'this' to God or his holy place."
Examples include:
Jacob gave a tenth of his earnings (Genesis 28:20–22).
The Israelites promised to give God the Canaanite war spoils (Numbers 21:2).
Hannah committed to giving her son as a lifelong servant to the Lord (1 Samuel 1:11).
Absalom offered sacrifices to God at Hebron (2 Samuel 15:8).
The sacrificial laws often mention rules for vows. Israelites made vows based on God answering their requests. They would then fulfill their vows to God, often by giving offerings (for example, Leviticus 7:16; 22:23; Deuteronomy 12:11).
Yet, people could take advantage of vows. The law itself warns against this (Deuteronomy 23:21–23). So do the wisdom teachings (for example, Proverbs 7:10–14; 20:25). Also, the books of the prophets (see, for example, Jeremiah 44:25).
King Saul often made foolish vows (1 Samuel 14:24–28). He did not keep the vows he made (19:6). People might not fulfill or repay careless vows. This could cause God to punish them. It is better not to make a vow than to break one (Ecclesiastes 5:3–5). Jesus commands us to live with such honesty and integrity that we do not need to make vows (Matthew 5:33–37; compare James 5:12).
Passages for Further Study
Genesis 28:20–22; Numbers 21:2–3; 30:1–16; Deuteronomy 23:18–23; Judges 11:30–40; 1 Samuel 1:11, 24–28; 14:24–45; Psalm 76:11; Proverbs 20:25; Ecclesiastes 5:3–5; Malachi 1:14; Matthew 5:33–37; Acts 18:18; 21:23–24; James 5:12