The short answer
The Eucharist is the sacramental gift of Christ Himself. In it, the Church receives the living memorial of His sacrifice and communion in His Body and Blood.
Jesus said
In the Bread of Life discourse and at the Last Supper, Jesus uses direct and covenantal language about eating His flesh and drinking His blood.
John 6:51–58Matthew 26:26–28Luke 22:19–20
The apostles taught
Paul identifies the Eucharistic cup and bread as participation in Christ and warns against receiving unworthily.
1 Corinthians 10:16–171 Corinthians 11:23–29
The Catholic synthesis
The Eucharist is the source and summit of Christian life because it is Christ's own self-gift made sacramentally present to His people.