Parable of the Workers in the Vineyard (Matthew 19:23-20:16)
Everyone is naturally content when other people treat him in a fair way. Unfortunately this view is not shared by everyone. One is quite used to that. But what in the case when God acts this way?
Jesus is telling a parable about how things works in his kingdom (Mt 20:1). He tells it in context of Peter's question: what there will be for them since they left everything for Jesus (Mt 19:27). Jesus answers them and in the end of his answer says that "many who are first will be last, and many who are last will be first" (Mt 19:30). These events are followed by Jesus' parable of worker in vineyard.
The parable is simple. In the end it shows up that the reward is not determined by the performance of the worker but by the will of landowner. Landowner wants to do it his way, because he wants to be good to everyone equally (20:15). It is not about who did the work best. The reward is here for everyone who accepted the invitation to work for him. That is what matters here.
The parable is trying to explain what Jesus actually meant by "first and last" in Mt 19:30. This is signaled by the grammar: preposition "therefore" (gr. γαρ) in Mt 20:1 signifying explanation; adverb "thus, this way" (gr. οὕτως) in Mt 20:16 signifying manner. Therefore the parable is illustrating meaning of this thought.
By explaining this principle of the Kingdom of God of "first being last and last being first" Jesus shows to his disciples genuine essence of top positions. None exists in reality. The greatest value is help realized in the service towards fellowman. Later he says to his disciples: "whoever wants to become great among you must be your servant, and whoever wants to be first must be your slave - just as the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many." (Mt 20:26-28).
You definitely want to read the story for yourself (see Mt 19:23-30 and 20:1-16).