The Sacrament of the Holy Eucharist is prefigured in the ritual sacrifices of the Old Testament, the Passover event, the manna in the desert, and the "thank offering" in the Temple.
The Eucharist was instituted by Christ at the Last Supper when he consecrated bread and wine, said they were his Body and Blood, directed his Apostles to eat and drink, and commanded them to "do this in memory of me."
We refer to the Real Presence of Christ in the Eucharist for the very reason that he is truly present, Body and Blood, Soul and Divinity, under the appearance of the consecrated bread and wine. The change in these elements, which takes place when the celebrant consecrates the bread and wine at Mass, is called transubstantiation.
"Eucharist" comes from the Greek word that means "thanksgiving." By partaking of the Body and Blood of Christ, we offer thanks for his one Sacrifice on the Cross, which freed us from sin and death.
The Eucharist strengthens and increases our union with Christ and our communion with the Church, increases supernatural graces and virtues, forgives our venial sins, strengthens us against temptation, and increases our solidarity with the poor.
The Eucharist, because it is a sign of communion with God and other members of the faithful, is a foretaste of Heaven.
We celebrate the Sacrament of the Holy Eucharist at every Mass. We are encouraged to receive Christ worthily every time we participate in the Mass.
(The Sacraments: Source of Our Life in Christ. The Didache Series).