And he said, “With what can we compare the kingdom of God, or what parable shall we use for it? It is like a grain of mustard seed, which, when sown on the ground, is the smallest of all the seeds on earth, yet when it is sown it grows up and becomes larger than all the garden plants and puts out large branches, so that the birds of the air can make nests in its shade.”
With many such parables he spoke the word to them, as they were able to hear it. He did not speak to them without a parable, but privately to his own disciples he explained everything.
Mark 4:30-34 ESV
This parable explains about the kingly rule of Jesus through his current ministry, it seems small and insignificant, some people have heard about it, some people have seen him, but only few responded to follow Him. One day the kingdom of God will be seen as the greatest kingdom of all, when all things will be revealed :)
Even now, we can see how Jesus’ ministry has grown, from 12 disciples, to the believers in all the nations across the earth, however everything will be revealed even more when Jesus come back :)