The poem is only a few lines long, but it sets two things against each other: a physical, early-morning act and the vague thinking the speaker dislikes. The speaker gets up early and heads to "the morning well," a concrete place where you draw something real and necessary. Then comes the blunt declaration: "I hate an abstract tract / made at night."
The epigraph, "No ideas but in things," helps explain the stance. The speaker seems to believe that thinking should stay tied to the physical world, to objects and actual experience. A "tract," a piece of argumentative writing, made at night suggests murky, overworked theorizing, cut off from daylight and anything you can touch. The rhyme of "abstract" and "tract" even makes the complaint sound a little clipped and sour.
The mood is fresh and plainspoken, with a touch of grumpy humor. Morning stands for clarity, while night stands for fuzzy thought. The poem argues for its point by doing it: it gives us a well, not a theory about water.