This exhortation refers both to attempting to "save" unbelievers by warning them of hell and to warning believers against the influence of apostates.
The ultimate hell (Greek gehenna) is not the same as the present hell (Greek hades), although eventually all those lost souls now in the latter will eventually be "cast into the lake of fire" (Revelation 20:15). Both are fearsome places of real fire. The inhabitants of Sodom, for example, have been "suffering the vengeance of eternal fire" (Jude 7) for thousands of years, though not yet in that ultimate hell. Also the rich man mentioned by Jesus was in Hades and yet was being "tormented in this flame" (Luke 16:23-24).
Both "hells" have literal fires, but it is hard to understand how material fires could torment non-material souls. There is a clue in James 3:6, which calls an unbridled human tongue "a fire, a world of iniquity: . . . set on fire of hell." Since the tongue is not literally on fire, but can be extremely destructive in human relationships, the implication is that hell itself is a "world of iniquity."
This aspect of hell makes it even more fearsome than literal fires could ever be. The existence there of billions of unredeemed souls, eternally separated from the holiness and love of God, where all who are "unjust" and "filthy" will continue forever to increase in their unrighteous and filthiness (Revelation 22:11), and in the constant presence also of the devil and his angels, is unspeakably appalling. Yet that was their choice when they rejected or ignored the infinite love of Christ.
No wonder that Jude urges us to warn them of such awful fire and seek to save them with fear if they won’t respond to the compassionate love of Christ. HMM http://www.icr.org/article/5997/