If a man vows a vow to the Lord, or swears an oath to bind himself by a pledge, he shall not break his word. He shall do according to all that proceeds out of his mouth. (Numbers 30:2)
When used as a noun, a vow is a promise. When used as a verb, vow means to promise something. So to “vow a vow” means to “make a promise”.
The CEB translation is:
When a man makes a solemn promise to the Lord or swears a solemn pledge of binding obligation for himself, he cannot break his word. He must do everything he said.
Keeping your word is an important principle in obeying God and is often emphasised in the Bible.