Hebrews 6:7-8 meaning

The Pauline Author gives an analogy here of soil that is fruitful versus soil that grows only thorns. He speaks of ground that drinks the rain which often falls on it and brings forth vegetation useful to those for whose sake it is also tilled (v 7). This first ground takes in all the rain delivered to it and brings forth vegetation that has value to the farmer. It is useful, so it receives a blessing from God (v 7). 

The second kind of ground yields thorns and thistles (v 8). It only produces prickly plants that do harm, so it is worthless and close to being cursed, and it ends up being burned (v 8). There is no fruit, no profit for the farmer, only weeds. The only use for the thorny ground is to cut it down and use it to fuel a fire. Perhaps the picture is of burning the field to clear it so it can be replanted with seed that bears fruit.

Believers can live a life where we receive God's will eagerly, using it to produce good things for other people. If we are faithful with what we're given, we will be blessed by God for our faithfulness. Or we can do nothing positive with the word from God, and produce sinful, harmful actions in our life. This is falling away. What we will produce in our life is of no use to God. It is worthless and it ends up being burned. The ground itself is not destroyed, but its worthless fruits are.

1 Corinthians 3:11-15 has a similar picture. That passage describes deeds done with a selfish motive as being like wood, hay, and stubble that is burned up in the refining judgment fire of Jesus. The ultimate goal for a believer's life on earth is to live such that Jesus is pleased with how we lived and will reward us with His blessing. 

Our time on this earth is the only part of our existence where we will be able to live and know God by faith. So what can be gained in this life can never be gained again. The immense loss will be unfathomable if we miss the incredible blessings God desires for us from our lives on earth. Conversely, the rewards God has for those who love Him by obeying Him in their walk are so enormous that we cannot imagine them (1 Corinthians 2:9).

The picture here is similar to the picture of the ground. How we live our lives matters: we can build a house of gems, gold, and silver (a fruitful, useful field), or we can build a house of wood, hay, and straw (a field that produces thistles and thorns). At the Judgement Seat, our works will be tested. What we did in our lives that honored God will be rewarded; we will be blessed. What we did that was useless or harmful or disobedient will be burned. We ourselves will be saved, but the fire will burn up our dead works.

We can miss out on being obedient, we can miss out on maturing into Sons and ruling with Christ in the kingdom. We are invited and urged to be fruitful, faithful, obedient — it depends on us to live our lives for God, or not. We cannot live a life pleasing to God on our strength alone; we need Jesus as our High Priest, and the Spirit as our Helper, but it's up to us to strive for maturity and to be obedient.

The Pauline Author says the unfruitful ground is close to being cursed. It is not cursed, but close. This is also similar to the language in 1 Corinthians 3:15: "If any man's work is burned up, he will suffer loss; but he himself will be saved, yet so as through fire." 

Any believer's justification in the sight of God is independent of any actions that believer takes, however bad. But this language indicates that we can squander the blessings to the point that we are barely better off. This is sobering language. Believers cannot out-sin the grace of God, but they can squander most of the benefits of God's grace.