This verse comes just after Jesus has told the parable of the sower to the crowds (Matthew 13:1-8) but before the disciples asked Jesus what it meant (Matthew 13:10).  Yet, by this short sentence Jesus was communicating a series of important truths.  Firstly, He was saying that the parable was not just a story for amusement but a teaching from God with a message to be understood.  Secondly, the meaning could only be discerned by people who had trained their hearts to understand what their ears heard.  Thirdly, some people would never understand because they would not accept the gospel.

Jesus told the Parable of the Sower (Matthew 13:1-8), but the crowds did not understand what He meant.  His disciples asked Jesus privately (Matthew 13:10), and He said that the parable was really intended for them, not the crowds (Matthew 13:11-13).  Then Jesus quoted almost directly from Isaiah 6:9-10 – initially a prophecy explaining why God sent His people into exile, over 500 years earlier, when they had refused to listen to Him.  Most of Israel’s and Judah’s kings, with their people, had been apostate.  They ignored prophet after prophet.  Their hearts were hard because they rejected

Jesus explained the Parable of the Sower to the disciples in private. They would have understood about sowing seed by hand.  It was not an accurate process to ensure that the seed only landed on fertile soil; some seed would go on the path.  Also, the Sower would not be alone.  Birds would flock to the field in the hope of a meal.  It would be more difficult for them to find the seeds in the crevices of the ploughed soil, but whatever landed on the path would be 'easy pickings' (Matthew 13:1-9).
 

In Jesus' Parable of the Sower, the third type of soil was full of thorns.  Scattering seed over that kind of land would be unproductive because the ground was not well prepared.  It was already occupied; any new seedling would have to fight for space to grow.  Jesus used this well-known agricultural scene to teach His trainee apostles about the reactions they should expect when telling the gospel to the world; and they needed to be prepared for apparent 'evangelistic failure'.