Mar 4:9 He that hath ears to hear, let him hear.
Christ repeats this phrase about eight times in Gospels. In Christ's use of symbols, people see with their eyes (physical), hear with their ears (mental), and understand with the hearts (emotional). This is a cycle that begins with seeing and hearing, the physical and mental. If we do not see or hear, we cannot understand (Mat 13:15). Christ originally introduced this parable saying that you must both see and hear (discussed here).
Christ uses this phrase to tell people that they must think about the true meaning of what he is saying. In other words, isn't obvious. He uses the phrase specifically when he talks about John being the second coming of Elijah, a very interesting idea and when he discusses his use of parables, and in general to refer to his use of parables or analogies.
There are several ideas that come together in this one phrase. First, that Christ's words require thought and analysis. When Christ wants to indicate that an idea is obvious, he uses the sense of sight to describe it. Things you see are obvious. When Christ talks about hearing, he is saying that it requires thought. Words are always symbols, but he uses symbols purposefully to force us to think, rather than jump to the conclusion that we understand what he is saying. Next, that everyone is capable of grasping it (everyone that can physically hear, that is), but at the same time suggesting that everyone doesn't always use what the ears and mental abilities that they have.
Interestingly, this was a common enough saying in Greek to appear under the definition of "ears" in the Perseus dictionary of ancient Greek and its use in Greek literature, that I primarily use for my research.