The Mass is both a sacrifice and a symbolic meal.
Let us first discuss the symbolism of the Mass. We note first of all the importance of meals in general: meals are occasions for loved ones to gather. In almost every culture, the sharing of a meal together is considered a special event. Furthermore, many cultures have their special meals--usually as a memorial to an important part of their history. For Catholics, the Mass is such a sacred meal, a gathering of our Christian family to celebrate Christ's saving Death and Resurrection.
We note, too, the symbolic elements of the meal itself: bread, water, wine. The unleavened bread and the wine were used in the Jewish Passover meal. Even more basic, however, is the ordinariness of these substances. Bread has been a nutritional staple in the human diet for centuries; wine was commonly used among ancient peoples as an additive to their impure drinking water and, of course, the drinking of wine is used to celebrate special occasions in many cultures. Water quenches our thirst and is used also used in cleaning. When we gather together for Mass, we all recognize the ordinary purposes of bread and wine; we should also understand how they were used in the a Passover tradition which Jesus celebrated at the Last Supper.
During the Mass, we offer the symbolic gifts to God through the action of the priest. In offering the bread, the priest reminds us that "it will become for us the bread of life." He mingles water and wine the "we may come to share in the divinity of Christ, who humbled Himself to share in our humanity." He then reminds us that the wine "will become our spiritual drink." Finally, before the bread and wine are consecrated, he washes his hands with the water, praying, "Lord, wash away my iniquity; cleanse me from my sin."
So we see that there is much symbolism and rich tradition in the Mass as a meal. But for Catholics, the Mass is more than just a symbolic meal. We believe the bread and wine become the Body and Blood of Christ at the Consecration of the Mass just as it did at the Last Supper. No longer are the bread and wine merely symbols which stand for something else; they are Christ Himself, although the appearance of bread and wine remains.
What about the question of sacrifice? Do Catholics believe that Christ is sacrificed again and again at Mass?
There is indeed a sacrificial dimension to the Mass. Remember that the Lord's Supper, our first Mass, was a Passover meal and the Passover recalled the night when the angel of death "passed over" those homes marked with the blood of an innocent lamb. Christians came to recognize Jesus as the Lamb-- anticipated in the Passover rite -- whose suffering led to our salvation. The Lord's Supper, which we celebrate anew at every Mass, turns our attention back to Calvary and forward to His ultimate coming.
As Paul wrote in the First Letter to the Corinthians, "as often as you eat this bread and drink the cup, you proclaim the Lord's death" -- which was indeed a sacrifice for our sins -- "until he comes." Reflection Paul's words, the Decree of the Second Vatican Council on the Church states: "Until the coming of the Lord, they (the priests)re-present and apply in the Sacrifice of the Mass the one sacrifice of the New Testament, namely the sacrifice of Christ offering Himself once and for all to His Father as a spotless victim."For further detailed study please refer to "Catechism of the Catholic Church" pages1356. 1357, 1362-1372, 1382-1390.