Interpretations
Most scholars think the parable was originally optimistic in outlook, in that despite failures eventually the "seed" will be successful, take root and produce a large "crop". It is the first parable to occur in Mark, which according to the Q hypothesis was the first book it occurred in. Mark uses it to highlight the reaction Christ's previous teachings have had on people as well as the reaction the Christian message has had on the world over the three decades between Christ's ministry and the writing of the Gospel.
Jesus says he is teaching in parables because he does not want everyone to understand him, only those who are his followers. Those outside the group are not meant to understand them. Thus one must already be committed to following Jesus to fully understand his message and that without that commitment one will never fully understand him or be helped by his message. If one does not correctly understand the parables, this is a sign that one is not a true disciple of Jesus. He teaches in this way so that their sins will then not be forgiven. He quotes Isaiah 6:9-10, who also preached to Israel knowing that his message would go unheeded and not understood so that the Israelites' sins would not be forgiven and they would be punished by God for them. Some debate whether this was Jesus' original meaning or whether Mark added this interpretation himself. The full explanation of the meaning of the parable stresses that there will be difficulty in Jesus' message taking hold, perhaps an attempt by Mark to bolster his readers' faith, perhaps in the face of a persecution. This parable seems to be essential for understanding all the rest of Jesus' parables, as it makes clear what is necessary to understand Jesus is a prior faith in him, and that Jesus will not enlighten those who refuse to believe, he will only confuse them.
The parable has sometimes been taken to mean that there are (at least) three 'levels' of divine progress and salvation.
Read more about this topic: Parable Of The Sower