This is a unique saying in the Gospels, attracting various interpretations.  However, in the context of the whole passage, Jesus is talking about the need to be preserved eternally.  It is an application and summary of most of Mark 9:30-48.  Those who deflect seekers from the Kingdom, are as much at risk of eternal loss (Mark 9:42), as those who indulge the flesh in unrepentant sin (Mark 9:43-48).  Jesus speaks about the fire of judgement which never goes out (Mark 9:48) from Isaiah 66:24 where the prophet describes the doom of rebellious spirits.  

But what about the disciples?  As we know, one (Judas Iscariot) was a rebel in his heart even while serving under Jesus (John 12:6): although he had every opportunity to repent, he did not.  The unquenchable fire awaited him.  How about the others, including Peter who denied His Lord, and Thomas who doubted His promise to rise again?  Jesus wanted them to know that 'fire' would come to all, either to refine in this life or punish in the next.  

In Malachi 3:2-4 God promised to refine the priests, like gold refined in a fire, so that they will be ready for His coming.  The trainee apostles would all be tested, as would the church, in persecution (Luke 21:12).  That hot testing was not intended to destroy their spirits; but rather to purify them by encouraging spiritual discipline and building up their reliance on God's grace.   'Salted with fire' reflects, 'You are the salt of the earth …' (Matthew 5:13).  The salt of truth and godly decisions is intended not just to preserve us in a good life now (and be a preservative in our society), but to preserve us (and those we minister to) for the life to come (1 Timothy 4:15-16).  Losing a grip on that truth would be like rain leaching out the soluble minerals from rock salt – only leaving useless stuff that could not keep anything preserved.  

While it is relatively easy to preserve a routine of religious habits, it goes against our human nature to preserve truth or to preserve our unity with our spiritual siblings (Mark 9:33).  The pressures, temptations and rivalries easily deflect us from our eternal destiny.  Discipline is important lest we allow the blessing to slip away from us, in a moment of self-will, like Esau (Hebrews 12:16).  Discipline cannot replace God's grace to sinners; but it does validate the sincerity of our relationship with the One who has been so gracious to us.  Alas Judas Iscariot heard the truth but replaced its importance with his own importance.  Neither did he care for the unity of the disciples.  The message to us is to be disciplined to hold firm to Christ the truth, hold firm in times of testing and hold firm together (Philippians 1:27).  Where we may be slipping, we must repent and return to following Jesus, today.