2. If a man vow a vow unto the Lord--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 because, referring to a sacred thing, it could not be neglected without the guilt of prevarication and unfaithfulness to God.
      he shall not break his word--literally, "profane his word"--render it vain and contemptible (Ps 55:20; 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 fulfilment. 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 unfavorable, released the devotee from all obligation [Nu 30:3-8].

JFB.