While they were eating, Jesus took bread, said the blessing, broke it, and giving it to his disciples said, “Take and eat; this is my body.”
For Catholics, the Eucharist (consecrated bread and wine given out at Mass) is the literal body and blood of Jesus Christ. It is not just a symbol but the actual body and blood of our Lord, as he told his disciples at the Last Supper.
During Mass, the priest consecrates the host (communion wafer or “bread”) and wine. The Holy Spirit enters into the bread and wine and it becomes the body and blood of Jesus. This process is called “transubstantiation.” This is why Catholics believe the Mass is the “source and summit” of Christian life; because we are being actually nourished by Jesus in his ongoing giving of his life to us!