What does John 6:20 mean?

The disciples followed Jesus' instructions to cross the Sea of Galilee. This obedience landed them right in the middle of the lake during a rough burst of wind. As should we all, the disciples are learning that following the will of God does not make us immune from hard times. Sometimes, in fact, it leads us right into stormy seas. In this case, Jesus has sent the men away from the crowds. This is to avoid entangling them in political unrest (John 6:15) and so they don't confuse the spiritual meaning of His miracle with the crowd's reaction. At this point, the men don't yet grasp the importance of what Jesus did in feeding the crowd (Mark 6:52).

This is not a hurricane, nor a typhoon, but the storm certainly makes getting across the water harder and more dangerous. But Jesus has been watching and decides to come out to assist them (Mark 6:48). In the middle of all that, the disciples see something which truly frightened them: a human figure walking on top of the waves. As it turns out, this is Jesus, who needs to call out to the men to reassure them not to be afraid. This is the fifth of seven miracles used in John's gospel to support the claim that Jesus is, in fact, God.

Matthew and Mark provide more details about this incident in their respective Gospels (Mark 6:45–52; Matthew 14:22–33). Among these elements is the fact that Peter attempted to walk out on the water towards Jesus, and briefly succeeded.