In Roman Catholicism, the Mass is a reenactment of the sacrifice of Christ on the cross in a ceremony performed by a priest. This ceremony is symbolically carried out by the priest and involves consecration where the bread and wine are changed into the body and blood of Jesus through the authority of the Catholic priest. When Jesus said, "Do this in remembrance of me," he gave the apostles and his future priests the power to change bread and wine into his body and blood, (Baltimore Catechism, Vol. 2, Q. 354).
The Mass in Catholicism is unbiblical. See the article, The Mass and the sacrifice of Christ.