Hatarat Nedarim (Annulment of Vows) is a widespread custom that deserves to be properly understood. For some people this is an opportunity to obtain release from obligations and commitments made during the year, and especially ones they've forgotten about, before standing before Hashem in solemn judgment. For others it's a chance to be a dayan, if only for a few short minutes. Here, in English, is a helpful and well-referenced account for you of what Rabbi Eliezer Melamed has to say about it in his Peninei Halakha (vol. 11, chapter 5) :
Hatarat Nedarim
There is a universal Jewish custom to annul vows on Erev Rosh
Hashanah in order to enter the holiday free from the serious sin of unfulfilled
vows (Shlah, Masekhet Yoma §2-4; Ḥayei Adam 138:8).
Some also annul vows before Yom Kippur. The recitation of Kol Nidrei at
the start of Yom Kippur is directed toward this goal as well.
This annulment is effective for vows that a person forgot
having made. It is also effective in annulling the obligation to continue
performing a positive custom that one performed three times without stipulating
that it does not have the force of a vow. It also annuls commitments to
voluntary mitzvot that one undertook without specifying that
it does not have the force of a vow. In contrast, if a person remembers making
a specific vow, he is not released from it until he details it before three
“judges,” who can release him based on a loophole and his regret (SA YD
228:14).
Since this annulment is not considered judgment, it may be
done at night, and the three people on the “court” may be related to one
another or to the person asking for the annulment. Accordingly, three brothers
may annul the vow of a fourth brother (ibid. 228:3). However, if a
woman requests a vow annulment, her husband may not be one of the three (ibid. 234:57).
The person asking for an annulment customarily stands, while
the three people granting the annulment sit, as a court would. Many people may
stand before the “court” at once and ask that their vows be annulled, and the
annulment may be extended to the whole group at once. Nevertheless, some are
careful to annul the vows of one person at a time (Mateh Ephraim 581:49).
If one dreamed that he made a vow, some say he should have
it annulled. Some even say that this annulment requires a panel of ten. Though
most poskim do not require any type of annulment for a vow
made in a dream, as it is not real, le-khatḥila we defer to the
stringent view (SA YD 210:2). If he cannot easily find a group of ten, he may
request an annulment before three, as one normally does for vows (Rema ad
loc.).
Women effectively annul their vows with the recitation
of Kol Nidrei. For this reason, they make sure to attend services
on the night of Yom Kippur. A married woman can appoint her husband as her
agent to annul her vows when he annuls his own. Since the two of them are like
one unit, when he stands before the “court,” it is as if she is with him.
However, an unmarried woman cannot appoint an agent (male or female) to act on
her behalf and annul her vows (SA YD 234:56; Taz ad
loc. 46; Rav Pe’alim OḤ
4:34).
