The story comes from the Bible. Specifically, Luke 15:11-32. Apparently the apostle Luke wrote the story down, but it was Jesus that was speaking. I should probably point out that, due to the popularity of this story, many people mistakenly believe that the word "prodigal" refers to someone who runs away from home. However, it actually refers to someone who spends exorbitant amounts of money without regard for the consequences. Sort of the opposite of a hoarder. Assuming you've read the story, you know that one of the boys demands his inheritance from his father. In modern times, that sounds like he wanted to strike out on his own, be independent, that sort of thing. In Biblical times, it was like telling his father, "I wish you were dead".
When he had spent all off his money the boy was forced to feed pigs for a living. He was even forced to eat from the pigs' trough. To a Jew, pigs were considered "unclean". I suppose they still are. Basically they were to be avoided. If they had contact with something unclean there were very specific rites that they had to go through to be "clean". So, eating and living and sleeping with pigs would be like, well, take the most disgusting thing you can think of. I'll leave that to your imagination.
Skipping ahead, when the son returns home, his father runs out to meet him. I'm told that the father's house would have been in the center of the town. So it wasn't as if the father looked down a long road and saw his son off in the distance. Also, I'm told that it was considered improper for a man of his status or age to run. Imagine President Obama giving a speech in a thong. That's the sort of embarrassment the father endured just to welcome his boy home.
When the father brings new clothes for his son and orders his servants to kill the fatted (meatiest) calf, this shows the father's capacity for forgiveness. The father in the story, of course, represents God.
I have to go to sleep now.