"Another parable He put forth to them, saying: 'The kingdom of heaven is like a mustard seed, which a man took and sowed in his field, which indeed is the least of all the seeds; but when it is grown it is greater than the herbs and becomes a tree, so that the birds of the air come and nest in it's branches."
Jesus gave us plenty of stories and parables to give us hints as to what the Kingdom of Heaven is. Here he relates it to a small seed, specifically a mustard seed, that grows into a large tree which gives place for the birds to nest and to live.
I believe that there are a couple lessons that we can learn from this particular parable. The first and most obvious one is that the Kingdom of Heaven is not one that will explode onto the scene. The Jews were expecting a warring revolutionary as the Christ. Someone who would come in with an army and defeat the Romans in a spectacular display of glory, but Jesus clearly had other plans. He didn't rally up a massive army to fight the Romans, or even a group of the smartest philosophers to affect people's thinking. Even in the end, Jesus didn't die a warrior's death with an army behind him fired up to continue the fight, He died a shameful death, nailed on a tree, in which even His twelve closest followers deserted Him at His greatest need. Even so the seed that He planted in His life quickly grew to be a revolution in philosophy, mindset, and religion that took the world by storm.
This is the same way that the Kingdom of Heaven affects each of us as an individual. It takes root through the simplest prayer of repentance. Nothing spectacular happens in that moment, most people don't even feel different, but as the roots begin to spread in a person's heart it grows into something revolutionary, all of a sudden people begin to notice that the person is completely different than they were before the prayer was said.
Finally, after the kingdom grows from a seed to a tree, it begins to sustain other life. As one seed of the kingdom grows it allows for other life to be supported and spreads life to those around it. If we look in the parable that Jesus gives we see that the birds are able to nest in the tree. That means that through the tree new life (little baby birds) is created. It is the same in our life when we allow the Kingdom to grow and develop in our hearts. Suddenly we allow live to be established all around us through the Kingdom in us.