Hebrews 6:4-6
4) For it is impossible for those who were once enlightened, and have tasted the heavenly gift, and have become partakers of the Holy Spirit, 5) and have tasted the good word of God and the powers of the age to come, 6) if they fall away, to renew them again to repentance, since they crucify again for themselves the Son of God, and put Him to an open shame. NKJV

Can someone explain this to me? What does it mean to fall away? Must the five conditions be met in order to be disqualified? Where is the line between this and a "prodigal" Christian who does a "re-dedication"? Or is someone who does this merely a person who never was a Christian but has only then been opened to Jesus?