Morning sunlight falls across a lawn, and the speaker treats what it does as a kind of lesson. The trees cast long shadows over the grass, and the poem describes them as if they were neatly drawn diagrams, "carefully tailored" and "serious," almost like a study set out for a student. Beside them, the flowers on the lawn make softer, less readable shapes, which the speaker finds less helpful.
What the speaker prefers is the clean line where light stops and shadow begins. That simple split, with its "interesting slant," pleases the eye precisely because it is not complete or exhaustive. The final line suggests that too much detail would spoil the effect.
The mood is calm, observant and gently humorous. The language is dry and a little formal, as though the speaker were a tidy-minded scholar admiring a garden. The poem seems to enjoy simplicity and the quiet pleasure of looking closely at something ordinary.