Four times (John 6:35, John 6:41, John 6:51, and John 6:58) Jesus says, "I am the bread of life." Like his conversation with the woman in John 4, Jesus uses their misguided desire for food to describe his mission.
His contrast with manna, which they had heard of and which their fathers ate, should have been clarifying, but it wasn't. It became difficult to deal with when he talked about eating his flesh and drinking his blood. Who wouldn't think he's a crazy-man? While many take those words sacramentally (they think he's talking about the Lord's Supper), it is much easier to understand that faith or belief is like eating. It is a metaphor for faith.
His claim of immense sovereignty is even harder to handle. Twice he says no one is able to come unless the Father draws them (John 6:44, John 6:65). They found this saying difficult because they were not being drawn and were unable to accept it. We must have a God who pursues us and drags us into faith, or we won't come!
The words "I will make him stand in the last day," tell us that nothing will ever defeat Jesus! Nothing! He will not fail in the end!
I love Peter's confession, "Where else will we go? You have the words of eternal life!" It has kept me in times of doubt. While I may not get everything, there is not a better alternative!