What we learn from the parable of the prodigal son is that God is very forgiving (perhaps more than would seem fair to us) and that God cares deeply about each person who goes astray.
We can see that God cares about each person in the way that the prodigal son is welcomed when he returns. His father does not scold him for being an idiot or wasting his money or anything like that. He receives him joyfully and throws a feast for him. This shows how much God wants us to come back to him when we stray.
We can also see that God is very forgiving. The younger son has been extremely foolish and has wasted what was presumably a large amount of money. Yet the father is not even angry at him. This forgiveness is so extreme that it makes the older brother (as it probably would make most of us) unhappy. This shows that God is much more prepared to forgive than most people would be.