The meaning of the parable is clear: Ancient Israel rejected prophets of old; they will reject and kill Jesus. Somehow God will rebuild on the rejected Jesus. How do we find a Gospel message relevant for people's lives? While there is some really interesting stuff in verse 34 about sending and bearing fruit, I sense myself drawn to verse 42 and the proclamation that God will rebuild on the rejected stone. This verse points toward the faithfulness of God, who rebuilds on Christ. I think we can apply this to people's lives: even through we again and again reject God's commands and even love in our lives, God rebuilds us on Christ.
Key Words in verse 34 -- which sets up the whole thing
ηγγισεν (from εγγιζω, meaning "approach", 21:34) This word, meaning "approach" or "come near", appears at turning points in the Gospel:
3:2 John Baptizes Jesus (John the Baptist say the Kingdom of Heaven is approaching/near)
4:17 Jesus begins ministry (Jesus says the KoH is approaching/near)
10:7 Jesus sends disciples (He instructs them to proclaim KoH is approaching/near)
21:1 Jesus is approaching/near Jerusalem
26:45/46 Jesus is betrayed (The hour is near/approaching)
What is interesting is that each time the Kingdom of Heaven is approaching, these is movement of Jesus and arguably a movement of the Spirit.
καιρος (kairos, meaning "season", 21:34) This word means 'season' or 'ideal time.' In this case, it describes the harvest season. It is always a reminder that in Jesus there is the fullness of time!
απεστειλεν (from αποστελλω, meaning "send", 21:34) I am amazed at how many times in Jesus' parables in Matthew we have (the character representing) God sending out people. I think we often think of this as a concept in John's Gospels, but it is really crucial to Jesus' ministry. We are sent out, certainly if and when the Kingdom of Heaven is approaching! This word is important because it reminds us that the Kingdom of Heaven, while principally about the movement of Jesus into this world and toward the cross (see discussion on ηγγισεν) it involves our movement too.
Other interesting words
οικοδεσποτης (oikos - despot, meaning "owner of the land", 21:33) This is a fun word in Greek. It is built on two smaller words that we can recognize: oikos and despot! It is interesting that Jesus would refer to God as an οικοδεσποτης, which was unlikely a favorable comparison for working-class listeners. It is a reminder that first, these are parables and not allegories; second, it all belongs to God.
εντραπησονται (future passive of εντρεπω, "respect", 21:37) The word for "respect" means more like embarrass...in short, they will be embarrassed enough to show respect. In the rest of the New Testament, it is always used within a context of shame rather than respect. Perhaps this is a reminder that respect within an honor/shame culture has a different meaning; perhaps it is a reminder that Jesus ends up shaming the pharisees and religious leaders. Ironically their attempts at shaming Jesus (killing him outside of Jerusalem) only lead to his glory!
οικοδομεω (meaning "build" "erect" or even "rebuild", 21:33 and 21:42) The word here for builder is the same as in vs. 33. God built something. People messed it up. God will rebuild. God is always at work revising the mistakes of our bad construction, relaying the foundation of our lives on Christ that we may bear fruit!
εθνει (ethnos, meaning "gentiles", 21:43) The word for "people" here is "Gentiles." Interestingly, Paul will talk about how he has a harvest of Gentiles in Romans 1:13, a case where Scripture is fulfilling itself!