In the celebration of the Eucharist, bread and wine become the Body and Blood of Jesus Christ through the power of the Holy Spirit and the instrumentality of the priest. The whole Christ is truly present -- body, blood, soul, and divinity -- under the appearances of bread and wine, the glorified Christ who rose from the dead. This is what the Church means when she speaks of the "Real Presence" of Christ in the Eucharist.

Where is the Eucharist mentioned in the Bible?
The Lord Jesus, on the night before he suffered on the cross, shared one last meal with his disciples. During this meal our Savior instituted the sacrament of his Body and Blood. He did this in order to perpetuate the sacrifice of the Cross throughout the ages and to entrust to the Church a memorial of his death and resurrection. The Institution of the Eucharist is written down in the four Gospels below: 

• Matthew 26:26-30
• Mark 14:22-26
• Luke 22:14-20
• John 6:22-59 (The Bread of Life Discourse)

Why does Jesus give Himself to us as food and drink?
Jesus gives himself to us in the Eucharist as spiritual nourishment because he loves us. By eating the Body and drinking the Blood of Christ in the Eucharist, we become united to the person of Christ through his humanity. "Whoever eats my flesh and drinks my blood remains in me and I in him" (Jn 6:56). In being united to the humanity of Christ, we are at the same time united to his divinity. Our mortal and corruptible natures are transformed by being joined to the source of life.

Is the Eucharist a symbol?
The transformed bread and wine are truly the Body and Blood of Christ and are not merely symbols. When Christ said “This is my body” and “This is my blood,” the bread and wine are transubstantiated. Though the bread and wine appear the same to our human faculties, they are actually the real body and blood of Jesus.

Sources: USCCB.ORG