Background:
In Luke 15 Jesus was teaching tax collectors and sinners. The Pharisees and scribes, the "religious experts" of Jesus' day, say that if Jesus truly were a righteous man, they reasoned, then He would not associate with such people; He would keep Himself pure and separate from sinners.
Jesus responded that,
"God rejoices more over the repentance of one sinner (those sitting with Him at the table) than over ninety-nine just persons which need no repentance" (Luke 15:7) and tells the 3 lost stories, the lost sheep, the lost coin, and the lost Prodigal son.

The Story:
The prodigal son is the longest parable in the Bible. A father has two sons; the older son is obedient to the father's wishes and stays at home to work with the family. The younger son is disobedient and asks his father for his inheritance, takes it and travels away from home. While this son is away from home, his inheritance is squandered, and he eventually ends up destitute, desperate and humiliated.

He realizes his mistakes and decides to return home to make amends with his father. When he returns home, his father rejoices in his return and throws a lavish party to celebrate. The older son is filled with anger, bitterness and resentment at the father's forgiveness of the younger son.

Some Points:

• By asking for his inheritance before his father died he is in effect wishing his father dead.
• The main point is that the father who represents God is full of grace for sinners who repent.
• Many think the story is as much about the older son, who could represent the self-righteous Pharisees, and does not have the forgiveness of his father.
  One of the duties of the eldest son would have included reconciliation between the father and his son. He would have been the host at the feast to celebrate his brother's return. Yet he remains in the field instead of in the house where he should have been. Yet, the father listens to his complaints and does not rebuke him.
  The older brother fails to realize that "anyone who claims to be in the light but hates his brother is still in the darkness." [1 John 2:9-11]

Links:
Parable of the Lost Son | Grace Communion International
What is the meaning of the Parable of the Prodigal Son?