This poem follows Li Po's seeming life quest to find enlightenment and beauty. The narrator makes a journey to find a reclusive monk in an old temple-- a fitting journey for life knowledge it seems.
However, there is disappointment, as with many of Po's poems, as when the narrator arrives the temple seems abandoned and is describe as dusty and empty-- not at all the sacred place expectations would hold.
However, after the dust is noted the narrator goes back several times and finally notices the beautiful mist (the anti dust...and eventual rain) surrounding the temple and all the glory around him.
This seems to be the key. The disappointment really is only the false expectation of what the monk was going to share with the narrator, some life changing prophecy or knowledge. That misunderstanding is washed away like so much dust as the narrator realizes through his journey and eventual patience that his answer is all around him near the temple. He is seeing what the absent monk knows through the surroundings.