The Feast of All Souls or All Souls Day is a commemoration of the Holy Souls in Purgatory. They are referred to as the Church Suffering. The existence of Purgatory is a doctrine of the Faith.
The Catechism of the Catholic Church states:
1030. All who die in God’s grace and friendship, but still imperfectly purified, are indeed assured of their eternal salvation; but after death they undergo purification, so as to achieve the holiness necessary to enter the joy of heaven.
Lesson 79 in My Catholic Faith says:
1. The poor sould cannot help themselves, for their time meriting was ended at their death. They cannot therefore merit anything to satisfy for their sins.
This is why we who can still merit by our good works should give some of them as sufferage for the poor souls, so that they may soon be delievered from their prison. We have the special obligation of helping with our prayers and sacrifices the souls of our dead relatives, friends, and benefactors.
Let us pray for the Holy Souls in Purgatory, for the release of their souls from the fires of Purgatory, that we may one day gain another friend in Heaven.