Matthew thirteen is a chapter which is filled with parables. Jesus was asked by His disciples, "Why speakest Thou unto them in parables?" (v. 10); "He answered and said unto them, Because it is given unto you to know the mysteries of the kingdom of heaven, but to them it is not given. For whosoever hath, to him shall be given, and he shall have more abundance: but whosoever hath not, from him shall be taken away even that he hath" (v. 11-12). In other words, those who have enough faith to become born again believers, their faith will grow; but those whose faith is too small, and is insufficient enough to produce salvation, even the faith they do have, will disappear.
Ironically, in a chapter of parables meant to disguise His message from those who opposed Him, the word "understand" appears five times:
 "Therefore speak I to them in parables: because they seeing see not; and hearing they hear not, neither do they understand."
 "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."
 "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."
 "But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty."
In all five instances, the word in the Greek is συνίημι (syniēmi), which means "to comprehend or believe." Notice, in verse 15, Jesus indicates that to understand with the heart, is what produces one's conversion (experience being born again). This idea is best illustrated in the first parable, The Parable of the Sower (13:3-9 w/ 18-23). Satan takes the "light" one gains from the Gospel, and "clouds the truth" (Gen. 3:1-5; Rom. 1:15-22; 2 Cor. 4:3-4; etc.). Note especially 2 Corinthians 4:4 which says, "In whom the god of this world hath blinded the minds of them which believe not, lest the light of the glorious Gospel of Christ, who is the image of God, should shine unto them." We either work for Satan, or we work for Jesus - it is our choice!
Jesus said, "As long as I am in the world, I am the light of the world" (Jn. 9:5). And again, "I am the light of the world: he that followeth Me shall not walk in darkness, but shall have the Light of Life" (Jn. 8:12). And finally, "Ye are the light of the world. A city that is set on an hill cannot be hid. Neither do men light a candle, and put it under a bushel, but on a candlestick; and it giveth light unto all that are in the house. Let your light so shine before men, that they may see your good works, and glorify your Father which is in heaven" (Mt. 5:14-16).