The Parable of the Workers in the Vineyard
For the kingdom of heaven is like a landowner who went out early in the morning to hire men to work in the vineyard. He agreed to pay them a denarius for the day and sent them into his vineyard.
About the third hour he went out and saw others standing in the marketplace doing nothing. He told them, "You also go and work in my vineyard, and I will pay you whatever is right." So they went.
He went out again about the sixth hour and the ninth hour and did the same thing. About the eleventh hour he went out and found still others standing around. He asked them, "Why have you been standing here all day long doing nothing?"
"Because no one has hired us," they answered.
He said to them, "You also go and work in my vineyard."
When evening came, the owner of the vineyard said to his foreman, "Call the workers and pay them their wages, beginning with the last ones hired and going on to the first."
The workers who were hired about the eleventh hour came and each received a denarius. So when those came who were hired first, they expected to receive more. But each one of them also received a denarius. When they received it, they began to grumble against the landowner.
"These men who were hired last worked only one hour," they said, "and you have made them equal to us who have borne the burden of the work and the heat of the day."
But he answered one of them, "Friend, I am not being unfair to you. Didn't you agree to work for a denarius? Take your pay and go. I want to give the man who was hired last the same as I gave you. Don't I have the right to do what I want with my own money? Or are you envious because I am generous?"
So the last will be first, and the first will be last. -- Matthew 20:1-16
As a child, I learned that this parable referred to the various periods of a person's life, when people are converted and enter into the service of God. Those who come to Christ at the eleventh hour, or even in a deathbed repentance, are to receive the same reward as those who have given their lives to Christian service.
I suppose that fits, and I don't know that this wasn't one of the things that Jesus had in mind, but I believe that it was intended to show that God has the right to distribute rewards however he sees fit, even to those who were introduced late into his vineyard.
In context with the latter portion of Chapter 19, I think that he was referring to the manner in which he may reward the Gentiles, who were the last called, or to the nations who shall be last converted to the faith, as he did to those who were converted by the apostles and the earliest preachers of the gospel. All of the faithful will be given their just rewards, even those who have labored but one our in his service.