Imagine a surprise birthday party. Obviously, the details are kept from the birthday boy or girl, but the whole reason that the party is kept a secret is so at the proper time the person may understand and appreciate everything that had gone on before.
Similarly in Jesus' ministry, he used parables so that his hearers would not understand. The crowds could not appreciate Jesus' mission and would not understand it. So Jesus commands people to be silent before his resurrection. Jesus' initial preaching was to bring judgment upon Israel, BUT after atoning for the sin of the world, the secret of the Gospel was to be revealed, or more precisely, had to be revealed!
But truly, the things of God are still hidden to those who do not believe in the Gospel, for in their unbelief they hear the Word of God but do not understand it. And this is why Jesus commands them in verse 24, SEE how you hear!
Jesus was commanding the people to evaluate and look at HOW they listened. How terrible would it be to be in the crowd and hear the words of Jesus yet not understand what he was saying? How foolish must those people have been!
How often do we hear the Gospel and not truly encounter it? How often do you come to church, hear what is being preached, but the Gospel does not cut you to the heart in worship and adoration of God?