Kol Nidrei is both the opening prayer and the name for the evening service that begins Yom Kippur. Kol Nidrei literally means "All Vows." It asks God to annul vows we may make (to God) during the coming year, either innocently or under duress. In other words, vows made unintentionally through the careless use of words or vows made because a person was forced to do so.
Kol Nidre was originally written and is still recited in Aramaic. It is said three times so that latecomers to the service will have an opportunity to hear the prayer. It is also recited three times according to the custom of ancient Jewish courts, which would say "You are released" three times when someone was released from a legally binding vow.
Kol Nidre must be said before sunset because it is technically a legal formula releasing individuals from vows in the coming year. Legal matters cannot be attended to on shabbat or during a festival holiday, both of which begin at sunset. As a result Kol Nidre is always recited before shabbat or the holiday begins. Although there have been attempts to eliminate Kol Nidre from the standardized prayer service, the melody is so beautiful that its emotional effect on listeners as become one of the most important aspects of the Yom Kippur experience.