# They will go out

Here "they" refers to all the people, the faithful Israelites and foreigners, who come to worship Yahweh.

# the worms that eat them

The worms represent the horror of decay and rot that are Yahweh's punishment on the wicked. 

# the fire that consumes

Fire also represents Yahweh's judgment. 

# will not be quenched

"will burn forever" 

# all flesh

This expression represents all created living beings that shrink from the dead.