What did Jesus mean when he said "let the dead bury the dead?"?

It's hyperbole.  He meant, "don't worry about that, follow me."

He meant that we all going to die , the one who is living today will be dead later .

He meant they would not accept Him as Savior of the world so they were the walking dead without hope of eternal life.  See God knows the beginning and ending.

MY taught's are those who do not believe in the word of GOD and the SON of GOD who shed HIS blood for the forgiveness of sins are already dead in their sins. 

This scripture is recorded at Matthew 8:21, 22 “Then another of the disciples said to him: ‘Master, permit me first to leave and bury my father.’ Jesus said to him: ‘Keep following me, and let the dead bury their dead.’” These words do not suggest against anyone’s attending a funeral. The man here addressed had said to Jesus that he wanted to go and bury his father, but that did not mean that the man’s father was dead already, for if his father had been dead at the moment the man would have been there at the bier and not listening to Jesus. But the man meant that his father was old and sooner or later would die; and so the man wanted to postpone following Jesus until after he had taken care of his father until death and could receive his father’s dying blessing and then fulfill his father’s wish to have his son at the deathbed close his eyes piously after death ensued. This would have delayed the man’s following Jesus indefinitely and hence Jesus said to let the dead bury their dead. The man’s relatives were evidently not following Jesus on the way to life and hence were dead spiritually and could be left to bury the man’s father when he died. Would you like to continue this Bible topic with one of Jehovah's Witnesses, or try out our free Bible study program. If so, please request a visit from one of Jehovah’s Witnesses by completing the form, a Witness in your area will contact you. 
https://www.jw.org/en/jehovahs-witnesses/request-a-visit/
Coronavirus (COVID-19) Alert: In many areas, we have suspended our in-person visits and public meetings. Please include your telephone number when you complete this form and a Witness in your area will contact you.

If u dun follow God, then u r dead.
If u say, i will do this first .. then God is not FIRST to u = wrong.

God is first in our life. Put Him first.b4 parents, love, etc.
Or u r useless .. as the dead.

Exactly that . Just be obeying all of Jesus' sayings .

At the time when Jesus spoke, all mankind were under the condemnation of death before God the Judge of all. So the ones Jesus referred to were evidently persons on earth who had been dead in a spiritual sense. Jesus must have referred to such spiritually dead ones when he said to the Jewish son who wanted to go home first to bury his father: “Keep following me, and let the dead bury their dead.”​—Mt 8:21, 22.

Jesus was referring to a group called minstrels.  They charge large sums to care for the terminally ill and serve as undertakers after their clients' death.

Only he could have known what he meant. Any guesses are speculations.