In "Effing the Ineffable", philosopher Roger Scruton discusses the undiscussable... that which is undiscussable not because of any distasteful content, but because it lies beyond the power of human language to express. Philosophers throughout the ages have, in the course of their philosophical journeys, come face to face with the ultimate, ineffable mystery of being, and have (in some cases at least) humbly given up the attempt to describe or explain it (while others, as Scruton playfully points out, have exhausted countless words in attempting to describe the indescribable).
Perhaps the most famous example of this reticence in the face of reality's ultimate mystery is the opening line of the Tao Te Ching (a favorite of my philosopher father): "The Way [Tao] that may be spoken of is not the eternal Way."
Scruton uses other examples, such as that of Thomas Aquinas, the builder of an immense, cathedral-like system of Christian philosophy in the 1200s, who ended up on his deathbed "in a state of ecstasy, declaring that all that he had written was of no significance beside the beatific vision that he had been granted, and in the face of which words fail."
There is also the modern example of Wittgenstein, who stated simply, "“that whereof we cannot speak we must consign to silence.”
As Scruton says, "There is nothing wrong with referring ... to the ineffable. The mistake is to describe it." The ineffable is that which is meaningful, but in a way that cannot be adequately expressed or captured in words. Often it is perceived in quasi-mystical experiences in which we might glimpse some deeper or fuller vision of the world, or perhaps an "intimation" (in Wordsworth's language) of another world beyond this one. Poetry is the closest that language can come to expressing such meanings, since poetry by its nature transcends the ordinary, practical use of language in order to express, or at least to suggest, the inexpressible.
Scruton gets high marks in my book for saying this about people who dismiss such notions of a transcendent reality as "unscientific fiction": "... people of this scientistic cast of mind are disagreeable to me. Their nerdish conviction that facts alone can signify, and that the “transcendental” and the eternal are nothing but words, mark them out as incomplete."