The poem is about how the same action can look like something else depending on who is watching. A thrush sees a man slapping himself, when he's really swatting mosquitoes. Cops see a woman who seems to be praying, though she's only closing her eyes to watch the afterimages. Both mistakes are harmless, even a little funny.
Then the poem turns. Admirers of a "legendary coach" see generosity in what he gave his boys and the places he took them. The poem doesn't spell it out, but the phrasing suggests that this kindness hides something much darker, and that the admirers are looking at it through the wrong frame. The light misreadings at the start make this one land harder.
The ending moves to the thrush's song, which we hear as mournful. The thrush may experience it as nothing more than dusk passing through its throat. The mood is quiet and uneasy: the poem asks how much we project onto what we see and hear, and what that costs when the stakes are high.