I was watching a show about kids in Korea and one of the segments featured a classroom that had a motto that said "The Future Belongs To Those Who Work Hard Today". Jesus says the work of God is this: to believe in the one whom God has sent.
The crowds finally found Jesus and he says to them basically you are only looking for me because I fed you. You want me to be your king so I can provide for you what you think you need for this life. What you really need is the kind of food which does not spoil which produces eternal life. They ask him then to show them a sign to prove that he is someone to believe in. For, their forefathers ate manna in the wilderness which was bread from heaven. But, it was not Moses who gave it to them. It was from God. Jesus says it is the bread of God that gives life to the world, and this bread is in the form of a person. He tells them, "I am the bread of life." Basically, I am what you need. Without bread/food we will perish. Without Christ we are finished. There are many in the crowd who are witness to this, but they do not believe.
Jesus continues saying, "All that the Father gives me will come to me, and whoever comes to me I will never drive away... I have come down from heaven to raise up all that the father has given me on the last day, and I shall lose none of them." We cannot come to Jesus unless the Father first draws us. If we are drawn to Jesus by the Father, and we believe in Jesus and come to him, we have everlasting, eternal life.
Jesus is the bread of life. He is what we need. Their forefathers needed to eat manna daily and they eventually died. Jesus says he is the bread of life. When we eat this bread we have eternal life. In other words, when we trust in Jesus we will truly have life. The sacrifice of his body on the cross was given for us, that we would be saved. We eat this bread symbolically when we celebrate communion. It represents the faith that we have in him, his sacrifice for us, and the fulfillment of this promise for us.