It probably means several things figuratively.
In the context of dealing radically with our sin (cut it off, pluck it out) of Mark 9:43-48, it means sin must die, or we must die; that if we are ruled by sin, we will be ruined by sin.
The worm which does not die could be the sinner's own conscience reflecting and reproaching, accusing and condemning itself.
The fire that is not quenched could be the wrath of God focused on a guilty and defiled conscience, with nothing to call upon to quench or mitigate the wrath.
To be noted is that the Jewish Pharisees, of which Paul was one, held that the wicked would be punished with perpetual punishment--to live forever dying, and to be forever in pains and griefs that never cease.
Isaiah 66:24 indicates that even when Gehenna will be destroyed, they will not be consumed, as it is written in Psalm 49:14: "death will feed on them," which the sages of the time commented on to mean "their forms shall endure even when the grave is no more."
And Jesus confirms this Jewish understanding in
Matthew 25:41 - everlasting fire,
Matthew 25:46 - eternal punishment,
Mark 9:43-48 - fire of hell (Gehenna) not quenched and worm does not die,
Matthew 13:50, 42 - fiery furnace, weeping and gnashing of teeth,
where Jesus sanctions the existing Jewish view of eternal hell, outlined in Matthew 18:6, Matthew 26:4; Luke 10:12.
Mark 9:48 is pretty unequivocal on this.
We get to choose whether to believe Jesus or not.
And we all get to deal with the consequences of our choices.