The Church teaches that those who die in God's grace and friendship, but are not yet completely purified, undergo purification before entering the fullness of God's presence. This purification is called Purgatory. Because the souls undergoing this purification can no longer gain merit for themselves, our prayers and the spiritual benefits of the Mass are of particular importance to them.
The Catechism of the Catholic Church reminds us that from the beginning Christians have honoured the memory of the dead and offered prayers for them, above all the Eucharistic Sacrifice, so that they may be purified and attain the beatific vision of God (CCC 1032).
There is no greater prayer we can offer for someone who has died than the Holy Sacrifice of the Mass. In the Mass, the one perfect sacrifice of Jesus Christ on Calvary is made present. Through this sacrifice, we commend the departed to the infinite mercy of God, asking that the merits of Christ's Passion be applied to their souls and that they may be brought more quickly into the joy of heaven.
Having Masses offered for our departed loved ones is much more than an act of remembrance. It is an act of faith, charity, and continuing love. Although death separates us physically, we remain united in the Communion of Saints. Our love for those who have died finds one of its greatest expressions in prayer for their souls.
So we should always remember our departed parents, spouses, children, relatives, and friends, requesting Masses for them, not merely on the anniversary of their deaths, but throughout the year. It's a precious spiritual gift, and perhaps the greatest act of charity we can offer to those who have gone before us.