He said, "This is how it is with the kingdom of God; it is as if a man were to scatter seed on the land and would sleep and rise night and day and the seed would sprout and grow, he knows not how. Of its own accord the land yields fruit, first the blade, then the ear, then the full grain in the ear. And when the grain is ripe, he wields the sickle at once, for the harvest has come." He said, "To what shall we compare the kingdom of God, or what parable can we use for it? It is like a mustard seed that, when it is sown in the ground, is the smallest of all the seeds on the earth. But once it is sown, it springs up and becomes the largest of plants and puts forth large branches, so that the birds of the sky can dwell in its shade." With many such parables he spoke the word to them as they were able to understand it. Without parables he did not speak to them, but to his own disciples he explained everything in private. (Mk 4:26-34)
Jesus gives parables about the kingdom. The first parable gives one of the important qualities of the kingdom: kingdom works on its own. Kingdom does not depend on human interventions and assistance. Human availability and openness are needed but for the kingdom to make itself realized in the world it does not need the help of man. Man cannot boast of having established the kingdom.
The second quality of the kingdom is that it appears to be small (insignificant) in its beginning but has greater influence and usefulness to humanity. Man should not think about the kingdom of God as he thinks about worldly/human institutions. These institutions have to impress people and show greater success rates etc to be of use. But the kingdom of God is totally of a different nature.
Jesus spoke in parables to the people in general. But to his disciples he explained everything to them so that they may understand the meanings. Explanations were given to the disciples so that they may in turn communicate the meaning to the people.