Jesus claimed to be the true bread of life come down from heaven. The expression "bread from heaven" was familiar to the Jews who in the desert received the manna from the hand of God. They ate it, but as Jesus pointed out, they all died. Jesus further told them, "The bread that I will give is my flesh for the life of the world." (Jn 6:51) The Jews were startled: How can he give us his flesh to eat? Jesus continued "Unless you eat the flesh of the Son of Man and drink his blood, you do not have life within you." (Jn 6:53) Aware that they were shocked, Jesus said to them, would you be convinced if you saw me ascending to heaven where I came from? "It is the spirit that gives life, while the flesh is of no avail. The words I have spoken to you are spirit and life." (Jn 6:63) They did not understand, so many disciples left him. The apostles also did not understand but continued to follow Jesus accepting him at his word.
This is the meaning of faith. We accept Jesus' words even if we do not understand because He is the Son of God who is truth itself. This is an act of humility on our part because we realize that our rational understanding is limited in comparison to God's infinite knowledge and wisdom.