פּירוש אויף דברים 23:24

מוצא שפתיך תשמר THAT WHICH HATH GONE OUT OF THY LIPS THOU SHALT KEEP — This is intended to add a positive command to the prohibition of delaying one’s vows (mentioned in the previous verse) (Rosh Hashanah 6a).

As you have vowed... to the LORD your God willingly If you said it willingly. However, if you were coerced and vowed it is nothing (legally insignificant), and it is written (II Chronicles 29:31) "and all who were of a willing heart brought burnt offerings." And our rabbis said (Chagiga 10a, and see Tosfot there on "לאפוקי", and Shevuot 26b) that the person who resolves in their heart needs to say it verbally, "and speaks truth in their heart" (Tehillim 16:2) - that they do what they had thought, like Rav Safra (Makkot 24a) - this is a pious trait, but one is not punished if they do not fulfil.

מוצא שפתיך תשמור, “that which has come forth from your lips you should observe and do.” “Observe” what you have vowed not to do and “do” what you vowed to do but make sure that you do this for the sake of Heaven.