Mark 4:30–32
30 And he said, “With what can we compare the kingdom of God, or what parable shall we use for it? 31 It is like a grain of mustard seed, which, when sown on the ground, is the smallest of all the seeds on earth, 32 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.”
Next we come to what is commonly known as the parable of the mustard seed. Again, Jesus uses a seed to describe the Kingdom of God. Here the emphasis is on the contrast between the size of the kingdom initially compared to what it grows into. The smallest of all seeds is a proverbial statement of the mustard seed (Literally it is not the smallest seed) commonly used in this culture of something small with great potential or ability to become something much more than expected. (see Matt. 17:20)
The main point is that the kingdom of God arrises from something insignificant and rather obscure, yet grows into something surprisingly large. Practically, the reign of God on earth will start as a small movement in Galilee and grow into the largest movement in all of history.
A tree with birds nesting in it is an occasional symbol in the prophets of the Gentiles being included in the people of God (see Ps. 104:12; Ezek 17:23; 31:6; Dan 4:9-21). So here Jesus is perhaps subtly alluding to the global reach of the kingdom of God to all people and nations of the world.
This certainly would have given the apostles hope as they follow Jesus around Galilee wondering when he would bring the kingdom in full, or even later when they were ministering and preaching throughout the book of Acts wondering why so many were rejecting the message. Jesus forewarned them of it's rather insignificant beginning, but encouraged them to persevere in the hopes of its later growth.