Jesus' purpose in telling the parable of the prodigal son was to demonstrate God's willingness to accept sinners who admit to their errors and show signs of repentance. The father's acceptance is contrasted with the older brother's resentment to show the difference between Jesus' and the Pharisees' attitude toward the repentant.
The parable begins with the son asking for his inheritance, a request that was considered improper. However, the father acquiesces and allows the son to leave, demonstrating the free will that God has given sinners. The son ends up squandering the inheritance and finds himself desiring the food of swine because of his poverty; this sequence symbolizes the idea often espoused by Jesus that distancing oneself from God does not improve a person's life.
The prodigal son reflects on the matter and admits his error in leaving the household. He returns and asks for forgiveness from his father, offering to become the father's servant; this symbolizes a sinner's repentance. The father, who has been waiting for his son to return, accepts him back unconditionally upon his arrival. This shows that God awaits a sinner's return and accepts him joyously upon his repentance.
The oldest son, used by Jesus to represent the Pharisees and scribes, is angry at the prodigal son's return. The father attempts to reason with him, but the older brother complains that his younger brother is being rewarded for having left. Similarly, Pharisees were annoyed that Jesus forgave and offered salvation to sinners. The father ends the parable by expressing his elation over the prodigal son's return, further emphasizing God's happiness upon receiving a sinner.