Four short lines, and nearly all the meaning sits in one odd image. The poem pictures a painter who has just listened to music. Afterward, the painter is compared to "an honest shadow / angry to hear of night." A shadow only exists because of light, so night, where shadows disappear, is a kind of end for it. Yet the shadow is angry just to hear about it, as if it had been caught off guard.
The title, "Lesson in humility," hints at what's going on. Judging by the imagery, music seems to show the painter something that painting can't do, or makes the painter feel small. The word "honest" matters here: the shadow doesn't pretend, and its anger feels truthful and a little wounded.
The mood is quiet, slightly rueful, and a touch humorous. The speaker seems to watch the painter with sympathy, noticing how a humbling moment can sting before it teaches anything.