Kol Nidrei (Aramaic: כָּל נִדְרֵי) is an Aramaic declaration recited in the synagogue before the beginning of the evening service on every Yom Kippur, the Day of Atonement.
Before sunset on the eve of Yom Kippur ("Day of Atonement"), the congregation gathers in the synagogue. The Ark is opened and two people take from it two Torah scrolls. Then they take their places, one on each side of the cantor, and the three (symbolizing a Beth Din or rabbinical court) recite:
"By the authority of the Court on High and by authority of the court down here, by the permission of One Who Is Everywhere and by the permission of this congregation, we hold it lawful to pray with sinners."
This invitation to outcasts is not specifically for Kol Nidre but for the whole of the Day of Atonement, it being obvious that when even sinners join in repenting, the occasion is worthy of Divine clemency. The cantor then chants the passage beginning with the words Kol Nidre with its touching melodic phrases, and, in varying intensities from pianissimo (quiet) to fortissimo (loud), repeats twice (for a total of three iterations) (lest a latecomer not hear them) the following words:
"All vows we are likely to make, all oaths and pledges we are likely to take between this Yom Kippur and the next Yom Kippur, we publicly renounce. Let them all be relinquished and abandoned, null and void, neither firm nor established. Let our vows, pledges and oaths be considered neither vows nor pledges nor oaths."
The leader and the congregation then say together three times, "May all the people of Israel be forgiven, including all the strangers who live in their midst, for all the people are in fault." (quoting Numbers 15:26.) The leader then says: "O pardon the iniquities of this people, according to Thy abundant mercy, just as Thou forgave this people ever since they left Egypt." And then the leader and congregation say together three times, "The Lord said, 'I pardon them according to your words.'" (quoting Numbers 14:20). The Torah scrolls are then put back in the Ark, and the customary evening service begins.
The vows and pledges being annulled by this ceremony are of a limited category. As the ArtScroll Mahzor explains it: "There is a dangerous and erroneous misconception among some people that the Kol Nidrei nullification of vows—whether past or future—... gives people the right to break their word or to make insincere promises that will have no legal force. This is not the case. The Kol Nidrei declaration can invalidate only vows that one undertakes on his own volition. It has no effect on vows or oath imposed by someone else, a court, or a gentile. Also, the invalidation of future vows takes effect only if someone makes the vow without having in mind his previous Kol Nidrei declaration. But if he makes the vow with Kol Nidrei in mind—thus being openly insincere in his vow—the vow is in full force. It refers to vows assumed by an individual for himself alone, where no other persons or interests are involved. "
Kol Nidrei is not a prayer, it makes no requests and is not addressed to God, rather, it is a juristic declaration before the Yom Kippur prayers begin. It follows the juridical practice of requiring three men as a tribunal, the procedure beginning before sundown, and of the proclamation being announced three times. (From Wiki)