Mark 4 discusses the Parable of the Sower and Jesus' explanation of it. Jesus tells the parable to describe how some people hear the word of God but do not understand or accept it, represented by different types of soil. He then explains that whether a person's heart is receptive or not depends on the individual, just as the productivity of soil depends on its nature. Jesus advises those who have heard the word to continue learning in order to better understand and spread God's message.