The Kol Nidrei prayer on Yom Kippur, is one of the most solemn prayers of the High Holiday liturgy. We are unable to enter the fast, without having our vows nullified.
Our complete slate is meant to be wiped clean on this holiest day of the year. In addition to praying for forgiveness for our sins, we also ask to be forgiven if we inadvertently made unfulfilled vows.
We begin this prayer by asking for permission to pray with the עבריינים, or transgressors. This is a highly unusual way to begin a prayer.
The ספר התודעה explains that this line was inserted from the period of the Annusim in Spain. These were Jews who practiced Judaism in private, while pretending to practice Christianity in public. They did this to save their own lives. They only came out of hiding on Yom Kippur.
Their choice of survival was not approved by the entire Jewish community. Therefore, permission was requested to pray with the עבריינים, the Annusim, when Kol Nidrei was recited.