What does John 21:4 mean?

Peter and six other disciples have returned to Galilee after Jesus' resurrection (Matthew 28:9–10; John 21:1–2). Most of these were fishermen before Christ called them (John 1:35–42). In one of His earlier lessons, Jesus directed Peter to make what seemed to be a pointless change in his technique: to cast the net on the other side of the boat (Luke 5:3–5). The result was a catch of fish so large it nearly sank two boats (Luke 5:6–7). Peter recognized that this wasn't just a lucky guess, but a supernatural miracle, and reacted accordingly (Luke 6:8).

The current fishing trip may have been meant to simply pass time, or to earn money. Either way, it was unsuccessful (John 21:3). As the sun comes up, they see a figure on shore, who calls out. What He tells them will echo Jesus' earlier instructions (John 21:4–5), and Peter will once again understand that it is the Son of God who is speaking (John 21:6–7).