Parashat Matot opens with a special command to the leaders of the tribes of Israel: "If a man makes a vow to Hashem or takes an oath to bind himself by a pledge, he must not break his word; he must do everything that leaves his mouth." This severe warning, which defines the concept of vows in Judaism in a single short verse with many interpretations, was spoken specifically to the heads of the tribes of Israel. It warns them that a person must understand that speech is not just something casual from the mouth outward. Instead, it is a gift from heaven that separates humans from animals and allows man to express himself through words rather than through exaggerated actions like animals.
The religious and believing person must remember that speech is a holy thing, and therefore, one must think and be careful before speaking. The Rambam wrote in a famous letter to his son, which some make sure to read every day after prayer: "And think about speech before you bring it out of your mouth," meaning to think carefully about what you want to say before saying it. In English, there is a wise saying: "Think twice before you speak once." It is also said that a person does not get a second chance to make a first impression, and until someone opens their mouth and speaks, everyone thinks they are wise.
Rashi's commentary on the command in the verse "he must not break his word" (lo yachel devaro) is that he must not make his words profane (chulin). This means keeping speech holy. Especially when a person makes a vow and promises to do something for God, such as a mitzvah, a donation to the Temple, or to a holy place, they must fulfill it and not break the promise, so that the things spoken in holiness do not become mundane.
