The image of the worm is taken from Isa. The two figures depict hell as a state of decay which is never completed and of burning which does not consume. For every one shall be salted with fire. Have salt in yourselves, and be at peace one with another. But the context here determines its meaning, for it has just been taken twice as a symbol of punishment.
Salt is a symbol of that which preserves from decay.
In both of these states the souls of men are salted or preserved. Though this state is a condition of life, it is such a negative and false condition that it is elsewhere termed a second death. This is the good state of preservation which a man is counseled to obtain, and not to lose, since it will not be restored to him. The passage summarizes and contrasts the two states of future preservation, one being the salt of eternal life which preserves a man to enjoy the love of God in heaven, and the other being the salt of fire which preserves him in hell to endure the just punishment of God. Even so it is not the will of your Father who is in heaven, that one of these little ones should perish.