Ok, Jesus is saying to the Apostles that even though they will see Him ascend bodily into heaven, it is the Spirit that will give life to the "bread' they eat. And it will be His life. And unless they eat this "bread" they will not have His life. The people who walked away upon hearing this teaching thought that He was preaching canabalism. But he explained to the Apostles that His body would ascend, yet they would still be able to eat His flesh because it is the spirit that gives life. This belief is not tangential. It is central to understanding Christ's mission of salvation.
And they'll never be hungry or thirsty again, right???