The Sacrament of the Holy Eucharist completes Christian initiation.  Those who have been raised in the dignity of the royal priesthood by Baptism and configured more deeply to Christ by confirmation participate with the whole community in the Lord"s  own sacrifice by means of the Eucharist.

At the Last Supper, on the night he was betrayed, our Savior instituted the Eucharistic sacrifice of his Body and Blood.  This he did in order to perpetrate 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 0f his death and resurrection: a sacrament of love, a sign of unity, a bond of charity, a Pascal banquet in which Christ is consumed, the mind is filled with grace, and a pledge of future glory is given to us.
Catechism of the Catholic Church 1322 and 1323

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Jesus is present with us in many ways—in Scripture, in prayer, in the poor and suffering. . . but He is present in a unique way in the Eucharist. The Catechism says that in the Blessed Sacrament “the whole Christ is truly, really, and substantially contained” (CCC 1374). Although all appearances (also called accidents) of bread and wine remain, the substance of both the bread and the wine are truly changed into the Body, Blood, Soul, and Divinity of Jesus. This is what the Church means by the term transubstantiation.  This change happens at the moment of consecration and continues as long as the Eucharistic species survive as such—in other words, until they are digested. Christ is completely present under both species, in the tiniest piece of a host and the smallest drop from the chalice. So even if you only receive under one species—only receiving the Host, or only receiving from the chalice—you are still receiving all of Jesus—Body, Blood, Soul, and Divinity. [from Presence: The Mystery of the Eucharist, Session 4 at Formed.org]