He shall not break ("yachel") his word, he shall do according to all that proceeds from his mouth (Num. 30:3)
The Hebrew word "yachel" is related to the word "chulin," meaning secular or worldly (the opposite of sanctified). From this we learn that a person should not only live up to and fulfill his official vows but abide by every promise and utterance he makes. (The Magid of Koznitz)
Reprinted from the Parashat Matot-Masei 5761/2001 edition of L’Chaim.