Numbers 30:2

If a man vow a vow unto the LORD, or swear an oath to bind his soul with a bond; he shall not break his word, he shall do according to all that proceedeth out of his mouth.

If a man vow a vow. A mere secret purpose of the mind was not enough to constitute a vow: it had to be actually expressed in words; and though a purely voluntary act, yet, when once the vow was made, the performance of it, like that of every other promise, became an indispensable duty-all the more that, referring to a sacred thing, it could not be neglected without the guilt of prevarication and unfaithfulness to God.

Shall not break - literally, profane his word; render it vain and contemptible (Psalms 55:20; Psalms 89:34). But as it would frequently happen that parties would vow to do things which were neither good in themselves nor in their power to perform, the law ordained that their natural superiors should have the right of judging as to the propriety of those vows, with discretionary power to sanction or interdict their fulfillment. Parents were to determine in the case of their children, and husbands in that of their wives-being, however, allowed only a day for deliberation after the matter became known to them-and their judgment, if unfavourable, released the devotee from all obligation.