The act of listening seems simple enough: the ears register the sounds produced and the brain interprets them, assuming the sounds reach the ears and the listener knows the meaning of the words. In the real world, however, the situation is usually far more complicated. First, to mentally process the message, the person to whom you are speaking has to be paying attention. Not only may external distractions—a baby crying or a TV on in the background—divert their minds away from the words, but their own thoughts might also similarly lead them astray. Lost in thought, they are just not hearing you.
Processing language takes a fair amount of thought. We use a short-term mental sketch pad, so-called working memory, to hold each word and its meaning in mind long enough to combine it with others. If the meaning of any of the words is unclear, the task becomes harder.