A raven passes over a cliff, and the poem catches the moment in a few spare lines. The bird's "great laugh" is its harsh, booming call, and the word makes it sound almost amused, as if it knows something the watcher doesn't. The speaker stays still, looking after it as it disappears, and that act of watching becomes a kind of "prayer." Nothing is asked for. The attention itself is the devotion.
The last stanza shifts into a stranger image: the raven's departure is "like burying daylight in the garden / where many eyes sleep." The light seems to sink out of the sky as the bird goes, and the garden of sleeping eyes suggests seeds, graves, or things waiting to wake. The poem doesn't explain it, which is part of its pull.
The mood is hushed and reverent, with a little eeriness. The speaker feels awe, and perhaps a quiet sense of loss, at something wild passing out of sight.