The Eucharistic Sacrifice. When the celebration takes place in church the Eucharist is the heart of the Paschal reality of Christian death. In the Eucharist, the Church expresses her efficacious communion with the departed: offering to the Father in the Holy Spirit the sacrifice of the death and resurrection of Christ, she asks to purify his child of his sins and their consequences, and to admit him to the Paschal fullness of the table of the Kingdom. It is by the Eucharist thus celebrated that the community of the faithful, especially the family of the deceased, learn to live in communion with the one who "has fallen asleep in the Lord," by communicating in the Body of Christ of which he is a living member and, then, by praying for him and with him.
Source: Catechism of the Catholic Church (Vatican.va)