Catechism of the Catholic Church §1182 — excerpt
The altar of the New Covenant is the Lord's Cross, from which the sacraments of the Paschal mystery flow. On the altar, which is the center of the church, the sacrifice of the Cross is made present under sacramental signs.
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.
Also cites: GIRM 259