18Then Jesus asked, "What is the kingdom of God like? What shall I compare it to? 19It is like a mustard seed, which a man took and planted in his garden. It grew and became a tree, and the birds of the air perched in its branches." – Luke 13:18–19
30Again he said, "What shall we say the kingdom of God is like, or what parable shall we use to describe it? 31It is like a mustard seed, which is the smallest seed you plant in the ground. 32Yet when planted, it grows and becomes the largest of all garden plants, with such big branches that the birds of the air can perch in its shade." –Mark 4:30–32
31He told them another parable: "The kingdom of heaven is like a mustard seed, which a man took and planted in his field. 32Though it is the smallest of all your seeds, yet when it grows, it is the largest of garden plants and becomes a tree, so that the birds of the air come and perch in its branches." -Matthew 13:31–32
The disciples said to Jesus, "Tell us what Heaven's kingdom is like." He said to them, "It's like a mustard seed, the smallest of all seeds, but when it falls on prepared soil, it produces a large plant and becomes a shelter for birds of the sky." – Thomas 20
The parable compares the Kingdom of Heaven to a mustard seed, which the parable says is the least among seed, yet grows to become a huge mustard plant that provides shelter for many birds. Much like the English saying Many an oak from a tiny acorn grows, and like the butterfly effect, the parable is usually interpreted as meaning that great things start from just tiny seeds of information, or from tiny actions.
It might also be interpreted to foreshadow the kingdom of Heaven growing forth from the small actions of the historical Jesus in life.