Yeshua tells the story of a man who has two sons. The younger demands his share of his inheritance while his father is still living, and goes off to a distant country where he "waste[s] his substance with riotous living", and eventually has to take work as a swine herder. There he comes to his senses, and determines to return home and throw himself on his father's mercy. But when he returns home, his father greets him with open arms, and hardly gives him a chance to express his repentance; he kills a "fatted calf" to celebrate his return.
Some Christians understand the story to expresses that the forgiveness of the son is not conditional on good works. Some interpret this story to mean that, when one comes to God, they should come with the intention to serve Him ("make me as one of your hired servants") rather than to make demands. However, most Christian theologians note that the story demonstrates repentance. All of this may be true enough, but this was not Yeshua's primary intent for telling the story.
In fact, the beginning of the chapter (Luke 15:1-2) tells us why Yeshua told this parable. "Now, all the tax collectors and sinners were coming near to him, to hear him, and the Pharisees and the scribes grumbled, saying, This one receives sinners and eats with them."
Yeshua used the parable to compare the Pharisees to the older brother of the "prodigal" son. Luke tells that the older brother, " was angry…and he said to his father: Look, these many years have I labored in your service, and never transgressed your command; and you never gave me a kid (young goat), so I might celebrate with my friends. But for this, your son, when he had dissipated your property with harlots, and came [home], you have slain the fatted calf for him."
Yeshua's point was that the Pharisees, like the older brother in the parable, were wrong to grumble against the sinners who sought out Yeshua, for, as the father said, "…it was proper for us to be merry, and to rejoice; because your brother was dead, and is [now] alive; he was lost, and is [now] found."