What does John 6:21 mean?

The gospel of John records seven specific miracles, which are described as signs—meaning events carrying some specific meaning, purpose, or message. In John's case, these are collected as evidence that Jesus Christ really is God incarnate. Two are recorded in this chapter alone: Christ feeding thousands (John 6:1–15) and Christ walking on the water (John 6:16–21). Here, however, there is another miracle which is described in an almost off-handed way.

Mark chapter 6 and Matthew chapter 14 give the details which John leaves out of this account. The men crossed the Sea of Galilee at Jesus' directive (Mark 6:45), encountering high winds and rough seas. Jesus, however, is watching from the shore, and walks out to them sometime just before sunrise (Mark 6:48). Even though the men are terrified at what appears to be a ghost, Peter works up the courage to walk on water, at least for a few steps, before being rescued by Jesus. The disciples then bring Peter and Jesus on board (Matthew 14:27–33).

This verse describes the "bonus" miracle of John chapter 6: when the disciples take Jesus into the boat, they are "immediately" at their destination!