What does Luke 12:21 mean?

Jesus finishes the parable of the rich fool. A member of the crowd swarming around Jesus and the disciples demands settlement of an inheritance issue between himself and his brother. Jesus refuses and tells a parable about a rich man who reaps a good harvest and decides to retire. That night, he dies, his riches go to another, and his soul is left without a relationship with God. This is the fate of someone who gathers earthly riches but does not spread the kingdom of God (Luke 12:13–20).

Next, Jesus will return His attention to the disciples. He has already spoken about having courage in front of authorities who can kill, knowing God protects the eternal lives of His children (Luke 12:4–12). Jesus will go on to encourage the disciples to submit their physical worries to God, like food and clothing—even giving those things up if it will serve the kingdom (Luke 12:22–34). They will need to sacrifice the time and work they could use to benefit themselves, remaining alert for His return (Luke 12:35–48). Finally, they may need to sacrifice their closest relationships if their family members refuse to follow Jesus (Luke 12:51–53).

Then, Jesus will put His full attention on the crowd, expanding on this message. He will tell them they can use signs to predict the weather, but don't notice that the kingdom of God has arrived (Luke 12:54–56). He will tell them to settle accounts with those they have wronged or they may find themselves imprisoned and broke (Luke 12:57–59). And He will expound on the need to reconcile with God. Like the rich farmer, no one knows when they will die. To concentrate on pleasures in this world at the expense of eternity is the most foolish choice of all (Luke 13:1–9).