It's the opening of the Dao part of the Dao De Jing.
Traditionally:
道可道,非常道。名可名,非常名。"The Way can be spoken, [but that] is not the constant/permanent/unchanging Way. The name can be named, [but that] is not the [you know] name."
Mawangdui:
道可道也,非恆道也。名可名也,非恆名也。[Same thing, but with more overt grammar, making it clearer that the first and third clauses say "The X is something you can Y" and not "The X that can be Yed". This doesn't make "The Dao that can be spoken" wrong, but that nuance is coming from knowing the meaning of the whole thing, not directly out of the grammar of the text.]
You can use words to describe it, but you'll fail to capture the whole thing.
In the traditional text, but not the Mawangdui text, 非常 has become a very common intensifier [literally "unusually"] in modern Mandarin. Classically it's two unrelated words.