You must fulfill what has crossed your lips and perform what you have voluntarily vowed to your God יהוה, having made the promise with your own mouth.
§ The Sages taught in a baraita: The verse states: “That which is gone out of your lips you shall keep and do; as you have vowed as a gift to the Lord your God, which you have promised with your mouth” (Deuteronomy 23:24). “That which is gone out of your lips”; this is a positive mitzva. “You shall keep”; this is a prohibition, as the phrase “you shall keep” is a warning to keep oneself from sinning. “And do”; this is an admonition to the court to make you fulfill your vow. “As you have vowed”; this is referring to a vow-offering. “To the Lord your God”; this is referring to sin-offerings, guilt-offerings, burnt-offerings, and peace-offerings, teaching that one must keep his word and bring them. “As a gift”; this is understood in its literal sense to be referring to a gift-offering. “Which you have promised”; this is referring to objects consecrated for Temple maintenance. “With your mouth”; this is referring to vows of charity, to which one commits himself with his mouth.
Kis összefoglaló a magyarázókról: https://chatgpt.com/c/6a4c2537-d524-83eb-8ffc-fb4f64c046fc
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מוצא שפתיך תשמר, "You shall carry out what you have promised with your lips." The Torah suggests that you should wait before making a vow, a promise, until you are in a position to honour it forthwith. In that event it is perfectly acceptable for you to make vows. This is why the Torah writes immediately adjoining ועשית כאשר נדרת, "and you will do in accordance with what you have vowed." The famous teacher of the Mishnah Hillel conducted himself according to this principle by not designating the lamb he would offer as a passover offering as such until he had reached the Temple yard where it was to be slaughtered (compare Pessachim 86).