Kol Nidrei is an aramic phrase meaning: all the vows.
The Tora warns against making vows lightly. Once a person made a vow in front of God he must fulfill its terms. If however there were circumstances that prevented the fulfillment of the vows, there are ways to retract the vows given to God. One such way is by reciting the Kol Nidrei before sunset on the eve of Yom Kippur.
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