"Then Jesus said, 'He who has ears to hear, let him hear.' When he was alone, the Twelve and the others around him asked him about the parables. He told them , 'The secret of the kingdom of God has been given to you. But to those on the outside everything is said in parables so that, 'they may be ever seeing but never perceiving, and ever hearing but never understanding; otherwise they might turn and be forgiven!'" NIV translation
Now, Jesus introduces us to the mystery of our spiritual life in Him. "He who has ears" was kind of a danger sign for them as it lets us know that not everyone will take His teaching to heart. The mystery for us is how we are able to hear and this speaks of the work of the Holy Spirit calling us and showing us the things of God. Jesus went on to quote from the prophet Isaiah and, when He speaks of the "kingdom of God", He is talking about both the earth and heaven.
"Then Jesus said to them, 'Don't you understand this parable? How then will you understand any parable?'" NIV translation
Although His questions to them may seem kind of harsh, Jesus is trying to get them to see the connection between the physical and the spiritual. His questions help us to see the importance of our change of worldview when we come to faith in Him. It is impossible to understand the things of God until that view is shifted to the spiritual instead of the flesh.
"'The farmer sows the word. Some people are like seed along the path, where the word is sown. As soon as they hear it, Satan comes and takes away the word that was sown in them.'" NIV translation
Jesus began to explain the parable to His disciples. We might ask how Satan can take the word from people and the answer is found in the soil. The soil represents the heart of man and the Word of God must not only be heard but it must penetrate the heart. The soil along any path is packed down and the seed cannot penetrate and be covered by the dirt. The soil (heart) must be prepared (broken up) to receive the seed (Word) and that is the work of God through the Holy Spirit.
"'Others, like seed sown on rocky places, hear the word and at once receive it with joy. But since they have no root, they last only a short time. When trouble or persecution comes because of the word, they quickly fall away.'" NIV translation
In order for a plant to survive and grow, water and nutrients must be taken in and this is the job of the roots of the plant. As the sun dries out the upper layer of dirt, the roots must be deep enough to get water that is left. When the ground has rocks instead of good soil, the roots cannot get down to where this supply of water is located and so the plant wilts and dies. Many have taken these words of Jesus to say that we can lose our salvation but that is not what He was saying.
Throughout the Bible, water is a symbol of the Spirit of God and, as we see here, it is this Spirit that sustains us through the hard times. This Spirit is a seal and a gift to those who believe in Him. If we simply grab onto the Word but miss the power of the Spirit, we can wilt and seem to fall away from the faith. In reality, our faith never had its roots in the good soil of Jesus Christ and so those that wilt never were actually saved to begin with. If our roots of faith are in Him, He will sustain us through the power of the Spirit and no amount of heat from the sun (persecution) can cause us to wither and fall away.