The father welcomes his wayward son back into the family. The parable is not saying the father is, or represents, God, but the situation does illustrate the way God treats a repentant sinner. The father now rejoices because the "dead" son is "alive". The Jews called the ungodly "dead", but in the New Testament it applies to those who have not responded to the kingdom message (the gospel) and who therefore cannot share in the resurrection-life of Christ.
The elder brother's reaction is very bitter. He claims his father hasn't even given him a "young goat", a fairly worthless animal. Like the wayward son, the stay-at-home son is also lost, lost in himself.
In describing his wayward brother, the elder son calls him "this son of yours." His father describes him as "this brother of yours" - an interesting twist.
Irrespective of the elder brother's tantrum, the father does not withdraw his love from either son.
Repentance and forgiveness bring fellowship and joy, but sadly, to only one son.
The parable of the Prodigal Son is one of those stories which has a locked-in interpretation. The parable is often used to teach the truth that God welcomes the repentant sinner. The story of a loving, forgiving, accepting father, illustrates the way God treats those who turn to him for mercy. Indeed, this idea has a central place in the parable, but there is more.
As verses 1 and 2 show, the parable addresses the muttering of the Pharisees who cannot accept the way Jesus "welcomes sinners and eats with them." In the parables of the Lost Sheep and the Lost Coin, Jesus makes the point that God joyously accepts those who come to him in repentance. In fact, he actively seeks after them and rejoices "over one sinner who repents." Jesus then goes on to tell the parable of the Prodigal Son so as to illustrate the way people respond to God's grace, his unlimited loving kindness. The wayward son repents and enters in to his father's joyous acceptance, but the righteous son stays aloof and so fails to experience his father's love.
So, some argue that the Parable is all about the elder son (the Pharisee, the self-righteous, rather than the younger prodigal son. It could be teaching us to accept the lost brother, the new convert, but rarely does a congregation not welcome a new convert. Some argue that it's all about welcoming the outcast, the poor, the oppressed within society and in the third world. For some the parable is all about forgiving and accepting a brother who has gone astray and may have hurt us in the process.
In truth, this parable is all about the two sons. One son lost to his father in body, the other lost to his father in mind. Both had free access to the father's unlimited bounty, both needed to rely wholly on his kindness, but only one chose to do so, and this the worthless son. Reliance on the free grace of God in Jesus Christ is the only means of accessing the fullness of his promised blessings. The rewards of eternity are wholly on the basis of God's freely given grace.