Hebrews 6:4-8 (NIV)
4 It is impossible for those who have once been enlightened, who have tasted the heavenly gift, who have shared in the Holy Spirit, 5 who have tasted the goodness of the word of God and the powers of the coming age 6 and who have fallen away, to be brought back to repentance. To their loss they are crucifying the Son of God all over again and subjecting him to public disgrace. 7 Land that drinks in the rain often falling on it and that produces a crop useful to those for whom it is farmed receives the blessing of God. 8 But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned.
Analogy
I came up with a little analogy that I think somewhat works (not perfectly):
Say someone, let's name him John, is driving a rented car and he goes to a car dealership just to see what there is. He finds a really nice car, so he take it out for a test drive and everything is perfect about it. After he returns, the sales rep tells him that he can have the car for free, he just has to accept it, no strings attached. There is nothing wrong with the car, but instead of accepting it, John decides that he doesn't want to deal with maintaining the car, and will instead continue with the rental because it's less work
He leaves, and the next day the dealership calls to tell him that the car is still available if he wants it, but he still rejects it. The dealership is persistent, but the calls become less frequent, and at some point John blocks the number. Over time the cost of the rental car is mounting, but he is adamant in rejecting the free car because it's too much work to maintain. Eventually he is left without a car and falls into ruin
Thoughts
I've heard it said that this passage, along with blaspheming the Holy Spirit, are both talking about people who understand The Gospel and have seen how good God is, and yet choose to reject God anyway and are continually unrepentant in coming back. Someone may turn away for a time, but if they are repentant, then this passage is not talking about them
It's like they "test drive" Christianity, and see everything good about it, and yet still choose to reject it. Even though people come to them and explain it in a better or different way they still think sin is better than "maintaining" the "easy yoke" of Christianity (Matthew 11:30)
They are continually offered the free gift, but have a hard heart and "block" any mention of it. Eventually God gives them over to the sin and it becomes impossible for them to return because they have continually been unrepentant. The gift is still offered, but they are set in their ways so much that to accept it at that point would be out of the question. They have produced thorns and thistles in their heart and don't want the good fruit that's offered
In the story of the 10 plagues, it says first that Pharaoh hardened his own heart, then towards the end God hardened his heart. Pharaoh was continually unrepentant and eventually God gave him over to the sin that he chose. He saw the power of God clearly seen, and yet was persistent in his rejection. So it became impossible for him to come back since he knows that his way is correct and nothing will change his mind.
However, as an encouragement, I don't believe this is talking about anyone that may wander away for a time but is repentant and genuinely wants God. Someone may have hard times and blame God, or maybe become indifferent to him for a time. We are all sinful but God is merciful and if we confess our sins, God will forgive us and cleanse our unrighteousness (1 John 1:9)
Remain faithful to the Lord because he is good