The Jewish people expected that the Messiah would come with signs and wonders greater than any of the prophets before him. Within the context there is conversation about the manna that came from heaven during the 40 year wandering of the Israelites before they entered the promised land. In verse 41 we read:
41 The Jews therefore murmured concerning him, because he said, I am the bread which came down out of heaven. 42 And they said, Is not this Jesus, the son of Joseph, whose father and mother we know? How doth he now say, I am come down out of heaven?
The comparison of Jesus to Moses is whats happening through out this section of verses. Jesus came down from heaven in the manor of manna. The thought is if Moses fed millions for 40 years and Jesus only fed 5000 for a day then Moses would be the greater prophet. But Jesus trumps Moses by saying that who ever partakes of the bread he has to offer shall not hunger again. He is being metaphorical about his body being bread. The idea is take and consume his teachings and his way of life.
J. W. McGarvey, The Fourfold Gospel: or A Harmony of the Four Gospels, (Cincinnati, OH: Standard Publishing Company, 1914)