The people are a bit confused: only the disciples had got into the boat on the previous evening but now in the morning, Jesus was gone. They did not know where, so they got into the boats and went to Capernaum to look for him. When they find him and ask how he got there he replies that they should not follow him only because he gave them bread to eat but that they should work for bread that lasts forever. This confuses the people. Here Jesus speaks directly to those who say that the kingdom of God should be built in this world, ie that if we had everyone fed enough etc all would be ok and everyone would be happy. Jesus says this is not enough, we have to work for an everlasting kingdom, and that he is the only one who can provide food for such a kingdom.
Finally, in the last sentence John sums up all of the Christian life. When asked by his disciples how to carry out the work of God, Jesus replies, "you must believe in the one he has sent." Such a simple answer to us who like to complicate things.....until we expereince that depth of the reply Jesus gives. Have you begun to experience that?!