Mar 4:23 If any man have ears to hear, let him hear.
Christ repeats this statement nine times in the Gospels. Most recently in Mark 4:9, which is discussed here, at the end of his first telling of this parable and right before the apostles asked him to explain it.
Here, the key vocabulary is the same. There is as slight difference of phrasing. Before explaining the parable, Christ says,""Having ears to hear, hear!" After explaining it, he says "If anyone has ears to hear, hear!" The final "hear" in both is imperative, a command, but the later is phrased more as a question. However, most of the difference in meaning comes from context.
Before explaining the parable, this statement was a challenge to his listeners to think about what he was saying and try to understand it. After explaining the meaning of the parable, Christ is asking the question. Do they now have more of an understanding of what he is saying?
Christ infers in this question that, even though he has explained his symbolism, many will still not understood what he is saying. Since he defines the seed on as "the word" and the various types of ground from preventing the word from yielding fruit, he knows that many questions must remain about the nature of the word and the type of fruit he is referring to.
However, in the previous verse, he said that everything that is hidden is hidden so that it can be understood. So despite the fact that he is realizes that understanding isn't easy, he has made it clear that it is possible. The point is that there is work to do. An idea that he continues in the next verse.