Be careful what it promises

And Moses spake unto the princes of the tribes of the children of Israel, saying, This is that which the LORD hath commanded.
When a man takes a vow to the Lord, or swears by binding his soul with obligation, he will not break his word; He will do according to all that came out of his mouth.

the word pledged is a commitment to fulfill

Source

"The word of a man is his obligation," says a saying. And this also applies to women! An "obligation" refers in this case to a guarantee or promise, a verbal agreement between two parties. Although a contract has not been signed, there is an understanding that a promise has been made and that there is an obligation to fulfill that promise

This was very true in ancient times when written contracts did not exist. Once a vow or oath was made, the trust implied in that promise could not be broken. In this passage Moses reminds the people that their promises to God and to other people have to be fulfilled. God ordained it and integrity demands it.
Unfortunately, we live in a time where words are spoken very carelessly and promises are made lightly. There are times when we move tempted to retract our words or get away from our commitments. But the word of God says that if we make a promise or enter into an agreement, we can not break our pawned word. If we miss our word, we betray the promises.

Be careful what you promise. It is better to pray before making a commitment. Knowing God's direction will prevent you from making commitments that you can not fulfill and will help you fulfill those you do.