Catechism of the Catholic Church §1055 — excerpt
By virtue of the "communion of saints," the Church commends the dead to God's mercy and offers her prayers, especially the holy sacrifice of the Eucharist, on their behalf.
This translation is under copyright, so only the cited words are quoted here. Follow the link for the full text. Translation: English translation of the Catechism of the Catholic Church, 2nd ed. (Libreria Editrice Vaticana), as posted at vatican.va.