This is the Sacrament where we receive the body and blood of Christ. As Catholics we believe that Jesus is truly present in the Eucharist.
In The Bible
The origins of the Eucharist are found in the Last Supper that Jesus shared with his Apostles. "In order to leave them a pledge of His love, in order never to depart from his own and to make them sharers in his Passover, he instituted the Eucharist as the memorial of His death and Resurrection and commanded his apostles to celebrate it until his return." (CCC. no. 1337)
With the institution of the Eucharist, Jesus gave the Passover its new and definitive meaning. He showed himself to be the High Priest of the New Covenant, offering himself as the perfect sacrifice to the Father.
The account of the institution of the Eucharist may be found in the Gospels of Matthew (Mt 26:17-29), Mark (Mk 14:12-25),Luke (Lk 22:7-20), and in the books of Corinthians(1 Cor 11:23-26).
"The Eucharist is the efficacious sign and sublime cause of that communion in the divine life and that unity of the people of God by which the Church is kept in being."-CCC, no. 1322 & 1325