"If a man makes a vow to Hashem... he may not nullify his words..."
The Talmud teaches that although one may not nullify his own vow, a sage
can find a method to nullify his words (Chagigah 10b).
A vow is a commitment taken upon oneself for self-improvement, such as not
to eat certain foods for a week or two in order to control one's desires.
This can be meritorious, but it is better to train oneself without using a
vow, saying "bli neder" (Rambam, Hilchos Nedarim 13:23-24). One who did
make a vow has a mitzvah to nullify it through a sage to avoid the risk of
transgressing it (ibid., 25).