This verse comes just after Jesus had told the parable of the sower to the crowds but before the disciples asked Jesus what it meant. 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 ears to know what they were listening to. Thirdly, some people would never understand because they would not accept the gospel.
 

Although Jesus spoke to many people, they did not hear Him or see what He was trying to show them. This was no new thing for Israel. Jesus was quoting almost directly from Isaiah 6:9-10 - which explained why God sent His people into exile over 500 years earlier - they had refused to listen to Him. Most of the kings had been apostate and they (and the people) ignored prophet after prophet. Their hearts were hard.
 

Everybody listening to Jesus 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'.
 

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. They would be responsible for telling the gospel to the world, and they needed to be prepared for apparent 'evangelistic failure'.