The Eucharist is the source and summit of the entire Christian life. Eucharist points to, all at once, thanksgiving, the sacrificial memorial of Christ and His body, and the presence of Christ.
"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 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.'" (Catechism of the Catholic Church, 1323)
As Catholics, we believe that the Eucharist is the Real Presence of Jesus Christ: His body, blood, soul and divinity. For this reason, we seek to worthily participate in the celebration of the Mass, where Jesus is truly made present through the priest in response to His words to the Apostles at the Last Supper: "Do this in memory of me."
The Eucharist is the source of our communion with God. It brings us into union with Him. A concerted preparation is necessary for this sacrament, because it is the presence of the living God: "Because this bread and wine have been made Eucharist ("eucharisted," according to an ancient expression), 'we call this food Eucharist, and no one may take part in it unless he believes that what we teach is true, has received baptism for the forgiveness of sins and new birth, and lives in keeping with what Christ taught.'" (Catechism, 1355)