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. The altar is also the table of the Lord, to which the People of God are invited. In certain Eastern liturgies, the altar is also the symbol of the tomb (Christ truly died and is truly risen).
Source: Catechism of the Catholic Church (Vatican.va)