"Jesus said to the crowds, this is what the kingdom of God is like." Two things. Note the word "crowd." Parables are open air events! Also, a comment on "the kingdom of God." The best equivalent for this word is "the Church."
This parable is the second parable of the Sower in Mark. We have seen in Matthew that the necessary thing for a Harvest is for someone to plant the seed. If we don't do that there is no harvest. "Night and day while we sleep, when we are awake, the seed is sprouting and growing." This happens of its own accord. It is a lovely thought to know that the growth is of "it's own accord." The Harvest is the Lord's. Our job is to plant the seed!!!!
The parable of the Mustard Seed is also in the other Gospels and we have dealt with it there. It is about small beginnings and great finishes. Do you remember that the mustard seed is the standard in Jewish law for the smallest measurable quantity? The passage concludes: "He would not speak to them except in parables, but he explained everything to his disciples when they were alone." We began by saying Jesus taught crowds by telling stories, but he explained everything later to his disciples. There is the big difference between teaching big groups and teaching small groups. We meditate on the parable to ponder the meaning, but we are taught one to one by the Holy Spirit of the deeper meaning.