Matthew 20:1-16 is the parable of the workers in the vineyard. Though each started to work in the vineyard at a different hour, at the end of the day they all received the same pay. The workers who worked longer hours were upset that those who worked only one hour received the same pay they did.
The general idea of the parable is that everyone comes to the vineyard (faith) at different points in their lives. Some people are lifelong disciples of Jesus, and some people convert only toward the end of their lives. In the kingdom of God, both groups are treated the same. By human standards, we perhaps might view this as unjust. Yet Jesus points out that the reward for their long work (heaven) is fair, and the only reason that others receive this same reward is because of the generosity of the owner of the vineyard (God). It is his (God’s) to do with as he pleases. So long as we have not been treated unjustly, how could we reasonably complain that God is generous to others?