We can find the parable of "the workers in the vineyard" in Matthew 20:1-16. Jesus doesn't explain the meaning of this parable to us so we'll have to work hard to understand it and pray that God would give us "the knowledge of the secrets of the kingdom of heaven" (Matt. 13:11).
Overview of the parable
The characters presented to us are a landowner and some hired workers. The image of a vineyard is often used to represent Israel or God's kingdom (Isaiah 5:1-7), and that is most likely the case here. This means that the landowner refers to God himself, and those workers in his vineyard (kingdom) are his people. God is the landowner. Christians are the workers. Some people become Christians very early on and some later in the day, but they all receive the same reward at the end of the day. The reward is not based on the work they have done, or how long they were involved - the reward is based on God's generosity. This presents a problem...
Some of the workers who have been there from the beginning complain against God's generosity to those who have only come at the end. They are complaining because it's very bad business practice! But God's Kingdom is not a business. It doesn't work on the principles of business... it works on the principles of grace*.
The big key verse for this parable is verse 16 "So the last will be first, and the first will be last." Jesus used this same phrase to finish off his teaching in the chapter before this parable (19:30) and it's a bit of a theme for this section of Jesus' teaching. It's a phrase that turns our thinking upside down, and one that we should remember, particularly when we think we're better than others.
What does this parable teach us about God?
This parable teaches us that God is generous. He does not give us as we deserve - he gives us more!
What does this parable teach us about His Kingdom? It tells us that God's Kingdom does not operate on the business principles of this world. God's Kingdom is not like an earthly kingdom - God's kingdom is about grace*.
What does this parable teach us about ourselves?
It teaches us that we are part of God's kingdom by his grace, and that the reward of eternal life with Jesus is a gift that is available for everyone who becomes part of God's Kingdom. Whether we become Christians early in life or late in life, we all receive the same free gift that God generously gives.
Special Note: Sometimes Christians are disappointed when someone becomes a Christian just before they die because they've lived their whole life in rebellion to God and done their own thing ... I've heard people say "it's not fair. They've been able to have all this fun and then become a Christian, but I've been a Christian my whole life, and missed out on all that stuff..." There's a real problem with thinking this way.
Firstly, if you think this way, you've forgotten that being a Christian is real life! Jesus came so that we may have life and have it to the full! (John 10:10). We should feel sorry for those people who have missed out on living a life in relationship with God, we shouldn't envy them for living a life of sin. That sort of life is really not life at all.
Secondly, if you think this way, you should be glad that they finally realised what you have known all along. And you should thank God that he is generous enough to save people that have lived their whole lives in rebellion to him. And you should be thankful that God has been generous enough to save you earlier on in your life rather than later. It is much better to live a whole life in relationship with God rather than only the last few years.
*Grace means: Undeserved favour. Receiving something good when you deserve the opposite.