Jesus often taught in parables. A parable is a short story of something ordinary that illustrates something spiritual. A couple of them are explained, but others require more thought to understand. The story of the farmer shows that some people are more open to trusting Jesus than others. It’s our job to spread the Word, but the result depends on the receiver. Jesus also uses illustrations of growth. The Jews were expecting a conquering Messiah, one who would arrive and turn things around in a single act of power. Jesus is letting them know God’s plan is different and involves a slower process of reproduction and growth over time.
As citizens of the Kingdom of Heaven, we are part of that growth process. We tell others of Jesus so they can become citizens as well.