The seed represents the Word of God. The seed (the Word) goes down to the people who hear it. Then the devil comes (the birds) and takes the Word out of their hearts, so that they don’t believe and don’t get blessed.
The seed, which falls on rocks, is the Word, which reaches people who hear it with joy, but the Word doesn’t have roots. For a while they believe in God but when someone makes bad remarks about their faith they renounce it and like rootless plants the faith dies.
The seed which falls under the thorns is the Word which reaches the people who hear the Word of God, but they have other interests (the thorns).  After a while they think more about money or have other problems. The Word of God doesn’t interest them any more then.
The seed which fall on good ground is the Word which reaches people who hear the Word of God and keep them in their hearts. They pass it on to other people so that there are more and more believers. With this action the Kingdom of God grows. (The Kingdom is not an actual country.)

Read online Bible:
Luke 8, 4-15
The Parable is a transfer of experiences from the world of work to the field of faith. Every listener knows how to sow to get a good harvest. The listeners think that it is outrageous that there is someone who acts so stupid when sowing.
Jesus explains to his listeners and to us the background, e.g. what happens to the Word of God, how people grasp it, and what then happens to the Word of God. He explains what is meant by the Kingdom of God.

Gospel according to Luke
The Parable of the Sower