The goal of Zen is the actual realization of ultimate reality. This realization called kenshō is really about our escape from the invisible cage of thought which includes language and concepts. Ultimate reality viewed from within this cage is not ultimate reality anymore than the finger pointing to the moon is the moon, itself. Kenshō is only possible if we can break out of this cage, staging a prison break so to speak. Those who have escaped from this cage are the only ones who know what the term "ultimate reality" is pointing to. These are the Buddhas.
For Zen Buddhism, the koan is about a prison break from the invisible cage of thought, including language and the concepts words evoke. In Zen, the signifier includes thought, language and concepts whereas the signified is the hua-tou 話頭, what is actually prior to thought, language and concepts being the real substance of the universe.
In the West things are a bit different. The signifier refers to a symbol, sound, or image whereas the signified is a concept which is distinguished from the sign. This difference is significant. It is all still within the cage earlier mentioned. That it is still within the cage is enough to make Zen almost impossible to understand from a Western perspective.
Looking at this matter further, we can see the difference between Zen and the West in Joshu’s ‘Mu’ 無. It begins when a monk asks Joshu, “Has a dog Buddha-nature or not?" Joshu answers, “Mu!” Mu means no. Both the question and Joshu’s response belong to the invisible cage. We could say that Mu is one of the bars. But as for the real Buddha-nature, it is beyond the cage. It is the hua-tou which is also true reality. It is before the arising of a single thought, word or concept.
This is where the Zen adept has to go. The adept is not interested in a conceptual interpretation of this koan through language. In fact, that is the problem. The koan has no answer other than personally to see the hou-tou.
Put another way, in order to know our Buddha-nature we have to break out of this invisible cage which is represented by Mu; which acts both like a barrier and a checkpoint 關 through which we cannot pass owing to the fact that we are attached to words and the concepts they evoke.
And would it be true to say that Zen is always showing us the invisible bars of this cage? Yes! But it is also saying that we have to escape from this invisible cage. We have to effect a prison break!