In the Gospel of John, Chapter 21, what do the following things symbolize:
The direction in which Jesus instructed the disciples to throw the net. ("Cast your net on the right side of the boat.")
The theme of this chapter in the Gospel of John is discipleship. When Jesus first called his disciples, many of whom were simple fishermen, he said, "Come. Follow me, and I will make you fishers of men" (Mark 1:17). Keeping this in mind, the story of the empty and full nets is a lesson in discipleship, obedience, and the fruit of the Christian faith, which is the "fishing" for and catching of souls for Christ.
Here, the disciples are fishing, but have caught nothing. At face value, the fish symbolize accomplishment, fruit, or evidence of faith. In the bigger picture of all the gospels, however, the fish represent the unsaved. The lack of fish at the beginning of the story symbolizes one of the key principles of Christian discipleship, which is that without Jesus, the actions of believers are fruitless (or futile). If the disciples were to go out and lead people to the knowledge of Truth, but did so without the Gospel message (which is Christ is Lord), they would not be successful.
The net in this story, as a tool for catching fish (or converting souls), could be symbolic of the Gospel message, but it could also be symbolic of the Holy Spirit, the gift given in the book of Acts just after Jesus ascended into heaven. The direction to throw the nets on the "right side" is symbolic of the simplicity of Jesus' directions, and the reward for obedience. Though Christ himself said in many of his teachings that following him would not be the easy choice, it can be argued that his direction or command to be a disciple is as simple as throwing a fishing net on the other side of the boat. He asks, simply, for obedience.