​Hebrews 6:4-8

  • For [it is] impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, If they shall fall away, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put [him] to an open shame. For the earth which drinketh in the rain that cometh oft upon it, and bringeth forth herbs meet for them by whom it is dressed, receiveth blessing from God: But that which beareth thorns and briers [is] rejected, and [is] nigh unto cursing; whose end [is] to be burned. (Hebrews 6:4-8)

I believe the falling away mentioned is talking about the hypothetical impossibility of someone losing their salvation (impossible because they can't lose their salvation) and getting it again, as that would be crucifying Jesus Christ again, which isn't going to happen.  Further, in the preceeding analogy, I believe saved people are the earth.  And I think it's referring to saved people because they were made partakers of the Holy Ghost.  I believe they're represented as the earth, because of the clear context, but also because of verses like these:
  • The first man [is] of the earth, earthy: the second man [is] the Lord from heaven. As [is] the earthy, such [are] they also that are earthy: and as [is] the heavenly, such [are] they also that are heavenly. And as we have borne the image of the earthy, we shall also bear the image of the heavenly. (1 Corinthians 15:47-49)
  • Then shall the dust return to the earth as it was: and the spirit shall return unto God who gave it. (Ecclesiastes 12:7)

Additionally, the rain is water, and water is often used to symbolize the word of God and the Holy Spirit:
  • That he might sanctify and cleanse it with the washing of water by the word, (Ephesians 5:26)
  • ¶ BETH. Wherewithal shall a young man cleanse his way? by taking heed [thereto] according to thy word. (Psalm 119:9) 
  • Jesus answered and said unto her, Whosoever drinketh of this water shall thirst again: But whosoever drinketh of the water that I shall give him shall never thirst; but the water that I shall give him shall be in him a well of water springing up into everlasting life. (John 4:13-14)
  • It is the spirit that quickeneth; the flesh profiteth nothing: the words that I speak unto you, [they] are spirit, and [they] are life. (John 6:63)
  • ¶ In the last day, that great [day] of the feast, Jesus stood and cried, saying, If any man thirst, let him come unto me, and drink. He that believeth on me, as the scripture hath said, out of his belly shall flow rivers of living water. (But this spake he of the Spirit, which they that believe on him should receive: for the Holy Ghost was not yet [given]; because that Jesus was not yet glorified.) (John 7:37-39)
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Then there are one of two things that believers can produce: herbs, or thorns and briers.  Herbs (good things) are produced by drinking in the rain (by reading the word and walking in the Spirit), and I take it thorns and briers (bad things) are produced if not.  I think God definitely, purposely allows us to see many interpretations of what symbols like this can mean in our lives, but, insofar as there are punishments and rewards in our earthly lives as well as the afterlife (even though salvation is free, simple, and can't be lost), I think that's what He's talking about.  Which is a result of our discipline or lack thereof in our doctrine, in our actions, in our words, etc.

Finally, as saints are the earth in this analogy, what's being burned is the thorns and briers, whatever those things are.  Not the earth, not the people in question. 

This should hopefully dispel the assertions of so many who claim this passage is about those who lose their salvation.