Our fathers did eat manna in the desert; as it is written, He gave them bread from heaven to eat.
Then Jesus said unto them, Verily, verily, I say unto you, Moses gave you not that bread from heaven; but my Father giveth you the true bread from heaven.
For the Bread of God is He which cometh down from heaven, and giveth life unto the world.
It seems the people wanted to debate Jesus. They asked what the work of God was, and He said, “Believe on Me!” But that wasn’t what they wanted. They were looking for a sign; they wanted more miracles, and they wanted those miracles to be to their benefit.
So they went in the direction of telling Jesus how their forefathers had eaten manna in the wilderness, bread given to them by God through Moses.
Jesus took a moment to remind them that it was not Moses who provided the manna; it was God, the Father, Who took pity on the people and sent this this daily gift of food from heaven. The people arguing with Jesus seemed to want the same thing from Him: Daily bread from heaven. The feeding of the previous day whetted their appetite for more.
In His response, Jesus tried to turn their attention from physical food to spiritual food–the Bread of God, sent from heaven to sustain them for all eternity if they would accept it.
He is necessary for spiritual life, just as bread is necessary for physical life and health. He made it clear that He was that Bread of Life, the Bread of God, come down from heaven to give life to the world.