"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. But land that produces thorns and thistles is worthless and is in danger of being cursed. In the end it will be burned." NIV translation
Now, Paul compares the Christian life to farming and they are closely related. Throughout the Bible, water is associated with the Spirit of God and, here, we see that a Christian that has received that Spirit (rain) should produce a crop. If farmland does not produce a crop over an extended period of time, then, it is no longer consider farm land.
"Even though we speak like this, dear friends, we are confident of better things in your case - things that accompany salvation. God is not unjust; he will not forget your work and the love that you have shown him as you have helped his people and continue to help them." NIV translation
Paul encourages his readers that they are saved and bases his confidence on the fact that their actions back up their words. He reminds them that God sees their work for Him and will remember it.
We see from this, that the falling away that he mentioned before was not losing salvation but basically being what is known as a pew sitter. So why do we not want to be a "pew sitter"? There are two reasons and the first is our love for God. Paul equates our love for God with our helping His people. If we love God, then we will serve people because He loves them. The second reason is that the loss that Paul is talking about is a loss of rewards. When we get to see Jesus, we will give an account of our lives after we accepted His grace. This is not a judgment with punishment but a sort of paying of wages. We will receive rewards from Jesus based on our service to Him on this earth.
"We want each of you to show this same diligence to the very end, in order to make your hope sure. We do not want you to become lazy, but to imitate those who through faith and patience inherit what has been promised." NIV translation
The hope that Paul speaks of is our rewards and he urges us to continue in the work of the Lord. People get lazy and want to kick back and wait for Jesus to return. Although we can do that, we will be missing out on some of the rewards that God has set for us.