[I have tried to find a separate group for this excellent parable, but unfortunately it appears that such a group does not exist, so I will leave this question in The Bible group, though of course this parable comes in "The Gospel of Luke" in the New Testament.]
When we think about understanding this parable, it is important to realise how it functions allegorically. On a literal level, the return of the younger son who has done so much to harm is father, and yet receives unconditional forgiveness, clearly suggests that those who repent their mistakes should be forgiven unconditionally. However, it is important to consider what various characters and aspects of the tale symbolise to unlock the meaning of the parable. We can safely assume that the younger son symbolises the human sinner. His employment as a swineherd represents the way that humans can become associated with unholy things (remember that pigs are considered unclean to Jews) and the elder son's anger and resentment clearly symbolises his envy at his younger brother. The long suffering father can be said to symbolise God.
However, it is also important to realise that Jesus said this parable to an audience that consisted of "sinners" and also the religious elite of his day. Indeed, it is in response to hearing the disapproving murmurs of the Pharisees and teachers of the law that Jesus tells this parable. Therefore we can speculate that Jesus is challenging their notion of forgiveness by placing the tax collectors and other sinners in the role of the younger son, and the Pharisees in the role of the elder son. The elder son expresses just the same kind of dissatisfaction and envy with the father and the Pharisees do to Jesus about his treatment of the sinners. Vital to the parable though is a sense of parallelism. Both times the father goes out to seek his son. However, whilst the younger son comes back and joins his father's house once more, the elder son is left arguing with his father in the fields. The response to the father's final words is left up to the Pharisees and teachers of the law. They can either join their father and make merry, or remain in the cold.