The Directory on popular piety and the Liturgy sums up the meaning of All Souls' day, which is today, in this way:
"The just encounter God in death. He calls them to himself so as to share eternal life with them. No one, however, can be received into God's friendship and intimacy without having been purified of the consequences of personal sin. "The Church gives the name Purgatory to this final purification of the elect, which is entirely different from the punishment of the damned. The Church formulated her doctrine of faith on Purgatory especially at the Councils of Florence and Trent".
Hence derives the pious custom of suffrage for the souls of the faithful departed, which is an urgent supplication of God to have mercy on the souls of the dead, to purify them by the fire of His charity, and to bring them to His kingdom of light and life. This suffrage is a cultic expression of faith in the communion of saints. Indeed, "the Church in its pilgrim members, from the very earliest days of the Christian religion, has honoured with great respect the memory of the dead; and 'because it is a holy and a wholesome thought to pray for the dead that they may be loosed from their sins' (2 Mac 12, 46) she offers her suffrages for them". These consist, primarily, in the celebration of the holy sacrifice of the Eucharist, and in other pious exercises, such as prayers for the dead, alms deeds, works of mercy, and the application of indulgences to the souls of the faithful departed".
Detail from the tympanum of the central west doors of Notre Dame de Paris, and my sermon for today, All Souls' day, can be read here.