The Parable of the Sower (Matthew 13:1-30)

What is the lesson of the parable of the sower?

"And in them is fulfilled the prophecy of Esaias, which saith, By hearing ye shall hear, and shall not understand; and seeing ye shall see, and shall not perceive: For this people's heart is waxed gross, and their ears are dull of hearing, and their eyes they have closed; lest at any time they should see with their eyes, and hear with their ears, and should understand with their heart, and should be converted, and I should heal them. But blessed are your eyes, for they see: and your ears, for they hear. For verily I say unto you, That many prophets and righteous men have desired to see those things which ye see, and have not seen them; and to hear those things which ye hear, and have not heard them." (Matthew 13:14-17) 

"Hear ye therefore the parable of the sower.  When any one heareth the word of the kingdom, and understandeth it not, then cometh the wicked one, and catcheth away that which was sown in his heart. This is he which received seed by the way side." (Matthew 13:18-19)