The resurrected Jesus showed himself to many disciples -- including Simon Peter, Thomas, Nathanael, and sons of Zebedee. The disciples then went out to do some fishing. At first, they couldn't catch any.
Eventually, Jesus, who was on the shore, told the disciples to cast the net on the right side of the boat. The disciples did not know that Jesus was telling them that, but they still did it anyway. Lo and behold, they caught a lot of fish! In fact, they at first struggled to carry the load because there were so many fish.
Eventually, the disciples were able to recognize that it was Jesus who told them to get some fish. One of the disciples told Peter, "It is the Lord!" (21:7). The fishing feat served as further confirmation that Jesus post-resurrection is indeed the same Jesus that the disciples were familiar with -- the one that did all sorts of crazy feats and important guidance.
Peter, however, had "plunged into the sea" in response (21:7). Considering that Peter had denied Jesus on three different occasions, it's likely that Peter still felt ippy from the shame after denying Jesus, so he tried to hide. But the shame wouldn't last long.