It concerned -- or named -- his theory that a poet doesn't need or desire to nail everything down, nor to understand anything through and through. That is, I take it, a poet acknowledges the impossibility of so doing, and writes within that limitation

The term you are looking for is "intuetics", inselpeter.** Nowadays it is referred to as "obscurantism" ... without any credit to Yeats. Defn. "A style in art and literature characterized by deliberate vagueness or obliqueness." [A-H]

And St. Paul said it best, long before Yeats, methinks. [Nostradamus and the priestesses of the Delphic Oracle had a leg up on Yeats, as well.]

to see through a glass darkly
To see “through a glass”—a mirror—“darkly” is to have an obscure or imperfect vision of reality. The expression comes from the writings of the Apostle Paul*; he explains that we do not now see clearly, but at the end of time, we will do so.

http://www.bartleby.com/59/1/throughaglas.html

* 1 Corinthians 13:12
For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.

**
"I still, however, tend to see the tower as a symbol of “reason,” reason as opposed to “intuition.” The rational thinking of scientists and scholars is continually opposed to the inspiration or intuition of the artist in Yeats’ poetry, as it was in almost all forms of romanticism. The tower is a symbol I will need to re-examine and come back to later. Maybe next time around I’ll argue that the tower symbolizes Yeats’ view that art should be practiced as an act of life, not removed from the world."

http://lorenwebster.net/In_a_Dark_Time/archives/000111.html