What does Matthew 13:11-12 mean? He replied, “Because the knowledge of the secrets of the kingdom of
  heaven has been given to you, but not to them. Whoever has will be
  given more, and they will have an abundance. Whoever does not have,
  even what they have will be taken from them. (Matthew 13:11-12)


What does it mean by that?
The basic idea is that when Jesus spoke in parables, they were great in drawing people with open hearts to him while driving those with hard hearts into deep confusion.  Parables were a common mode of teaching among the Rabbis, but Christ used them in such a way that heaven shined down upon the most mundane every day practices of life. He also used them almost exclusively, which was not normal at all. Especially the more people began to reject him, the more he spoke strictly in parables and his parables took a greater tone of warning. They were deeply illuminating to the heart open to God. They were foolish nonsense, provoking mad frustration to those hard of heart. Therefore, they had a winnowing effect. They divided people. Some were left asking questions, seeking and knocking, others left stumbling and cursing questions.

In general, Christ's parables spoke about the nature of his kingdom.  How one was in it, or not, based on their nature, which would eventually show itself good or bad. Everything was about the nature of a person, not their works, but a good nature would grow up good, and a bad nature grow up bad. To enter into this kingdom of life, one had to believe in Christ, receive his seed, believe his word and be a new creature. In the end, the bad creatures, weeds, fish, would be tossed, burned, weep and gnash their teeth, etc. The good creatures by virtue of their being in the kingdom, would be congratulated, rewarded, etc. The language was vague enough to confuse anyone who was self righteous not knowing they needed faith and re-birth, but anyone who longed for forgiveness and felt the strings of their heart moved and warmed by Him whose words were eternal life, the same  were drawn in.

We can see this play out as when Christ started to say things hard to understand and receive, such as the need 'to eat his flesh', it was too much for many and they left him. The disciples were also confused but their hearts were drawn in and they could not leave him.


  From this time many of his disciples turned back and no longer followed him.
   “You do not want to leave too, do you?” Jesus asked the Twelve.
   Simon Peter answered him, “Lord, to whom shall we go? You have the words of eternal life. (John 6:66-68, NIV)


The same thing is at play when Jesus spoke to a religious leader who seems a bit on the fence between having what he had taken away, or entering into what he had being multiplied into new eternal rebirth:


  Flesh gives birth to flesh, but the Spirit gives birth to spirit. You should not be surprised at my saying, ‘You must be born again. ’ The wind blows wherever it pleases. You hear its sound, but you cannot tell where it comes from or where it is going. So it is with everyone born of the Spirit.”
   “How can this be?” Nicodemus asked. (John 3:6-9, NIV)


Again Jesu spoke to a woman at a well who is on the fence and being drawn in.  Jesus implies that what she has will be multiplied into living waters of new eternal life:


  Jesus answered, “Everyone who drinks this water will be thirsty again, but whoever drinks the water I give them will never thirst. Indeed, the water I give them will become in them a spring of water welling up to eternal life.” (John 4:13-14, NIV)


In other words that which was a blessing to those open to God became judicial in judgment to those who were not open to God.  Like a magical filter his parables bought his sheep into the gate, and closed it in confusion to those who were not his sheep. His sheep were introduced into the gospel obtaining an eternal new birth. His enemies had no idea of anything he said and thought it was just meaningless babel. Even the form of knowledge they had of God was taken away when rejecting Christ, leaving them to cross their eyes is utter confusion.