I am the living bread that came down from heaven; whoever eats this bread will live forever, and the bread that I will give is my flesh for the life of the world. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him on the last day. For my flesh is true food, and my blood is true drink. Whoever eats my flesh and drinks my blood remains in me and I in him.
John 6:51, 54-56
At the Last Supper, on the night before he was betrayed, our Savior instituted the Eucharistic sacrifice of his Body and Blood. This he did in order to perpetuate the sacrifice of the cross throughout the ages until he should come again, and so to entrust to his beloved Spouse, the Church, a memorial of his death and resurrection: a sacrament of love, a sign of unity, a bond of charity; a Paschal banquet in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us.
Constitution of the Sacred Liturgy Number 47
Second Vatican Council
The Eucharistic celebration always includes the proclamation of the scriptures, especially the Gospel; thanksgiving to God the Father for all his benefits, above all, the gift of his Son; the consecration of bread and wine; and participation in the liturgical banquet culminating in receiving the Lord’s Body and Blood.
The essential signs of this ‘Sacrament of sacraments’ are wheat bread and grape wine, on which the blessing of the Holy Spirit is invoked and the priest pronounces the words of consecration spoken by Jesus at the Last Supper. By the consecration the transubstantiation of the bread and wine into the Body and Blood of Christ is brought about; Christ himself, living and glorious, is present in a true, real, and substantial manner, his Body and his Blood with his soul and divinity. The Church warmly recommends that the faithful receive Communion when they participate at Mass.
Catechism of the Catholic Church Numbers 1342, 1408-1409, 1412-1414, 1417