Verse 1

The Tao that can be told
is not the eternal Tao
The name that can be named
is not the eternal Name.

The unnamable is the eternally real.
Naming is the origin
of all particular things.

Free from desire, you realize the mystery.
Caught in desire, you see only the manifestations.

Yet mystery and manifestations
arise from the same source.
This source is called darkness.

Darkness within darkness.
The gateway to all understanding.
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Interpretation:
There are three parts to this seminal verse, each successive part (loosely a paragraph) builds upon the former...

FIRST PART:
The first part discusses how the Tao (or the Eternal Way of the Universe; or maybe even interpreted as part of, or a manifestation of, God) is something that can never be fully described.  Our words are imperfect.  Description of a thing is not the thing itself.  This is common sense but must be said because it relates to non-attachment to the material world.  It also relates to our tendency to make artificial distinctions and then put a lot of stock in our artificial distinctions (like beautiful versus ugly or good versus bad).  These are merely approximations to the divine concepts and one should not get overly attached.
Other examples of this distinction between divine concept and physical manifestations are:
     - Thomas Aquinas, The Uncaused Cause proof of God makes a distinction between that which is    manifest and that which is unmanifest.  The first cause cannot be manifest because it manifests the universe.  The creator cannot be made of the same stuff as its creation.  In other words, God cannot be an effect of some other cause.  That is why God is the UnCaused Cause.  Thus, the God that can be described by its manifestations is not the eternal God.
     - Plato's Pure Forms distinguishes between a generic, "perfect" concept (like the perfect horse) and particular instances we see in reality (like a Mustang named Penny or a Clydesdale that lives in the neighbor's farm).  "A Form is an objective 'blueprint' of perfection" (from the wikipedia article referenced via the link).  Thus, the Thing that can be Named is not the Pure Form of the Thing.
     - Jung's Archetypes also relate to the notion of common, universal concepts that are shared among all humanity but are abstractions of the physical, or particular manifestations.  Thus, the archetype that can be named is not the Universal Archetype.

Thus we have many good examples of the distinction between an abstract concept and a physical manifestation.  Of course, besides the abstract/physical distinction is the notion that our descriptions will fall short of the "pure" or "perfect" form.

SECOND PART:
After establishing that the eternal Tao, or way, is something that we cannot describe perfectly.  Lao Tzu seeks to leverage that lesson to discuss the next lesson which is that not only is the Tao indescribable but that it embodies mystery.  In other words, all of the Tao is not meant to be known.  There is mystery in the infinite. Just look at the vastness of the stars up above and it is easy to realize how unfathomable the universe is.  How many life forms exist?  How many stars are in the universe?  How did all of this amazing creation come to exist?  Why does it exist?  Many mysteries are there for us and will always be there.  We will never solve all the mysteries ... AND THAT IS OK.  Mystery is part of life.  Not knowing everything is part of life.  The desire to know everything is wrong.  And that leads us to the state of desire...  As is described in the Second Noble Truth of Buddhism:  Greed, or desire, is the Cause of Suffering. So, here Lao Tzu is saying that if you want to understand the Way, you must be "Free from Desire".  When you are interested in understanding the truth, in seeking with a pure heart, you will see the Tao as mystery and delight in that mystery.  As the Bhagavad Gita states, you must renounce the fruits (results) of action.  You act for the sake of doing what is right; acting without desire for reward; acting with your full heart and mind.  Only then will you see the mystery as a good thing.  Otherwise you are attached to the result, you are attached to the manifestation and can only see the manifestation.  That is not the Way (Tao), not the path to Enlightenment (in Buddhism), not the Path to Freedom (in Hinduism) and not the Path of Righteousness (in Christianity).

THIRD PART:
Now that the teacher has shown us that we will be discussing the "Pure Form" of "The Way"; and though imperfect, we must approach it "Free from desire" to see its mysteries; Lao Tzu completes the discussion by stating that the UnManifest and the Manifest all come from a single source.  As Genesis states, God said, "Let There be Light" and it emanated from a single point.  The Big Bang theory also asserts that the universe began from a single point.  A single point, a single creator, a single source of all that is Manifest and UnManifest.  But many of these things are "hidden" from us in the "darkness".  In fact, even more than that there is "The Darkness within Darkness".   In other words, there are layers of mystery and that is good.  Again, back to Genesis, and God saw that it was good.  Or in the words of Lao Tzu, the darkness within the darkness is "the gateway to all understanding" (which is a good thing).  It is very important here to understand that darkness does not mean evil here, but obscurity.  My brother Joseph quoted from the Bible,  "God dwelleth within the thick darkness" [1 Kings: 6-12, Exodus] ... as Christians, you cannot refer or represent darkness as evil here.  Darkness here (in this verse) means obscurity - something that is hidden in the deep.  Something that is hidden and mysterious.  Something that may never be fully known.  Something that is indescribable.  These mysteries are what the Tao is about.  Though it may not describe them perfectly, it will describe their secrets and empirical manifestations in 81 ways.  As Jesus said many times, "He that hath ears to hear, let him hear."