The speaker here is a dog, and the poem is about losing the person who guided it. The dog doesn't understand death, so it reaches for the closest explanations: the man has "turned into a bird or some other / uncatchable thing." That's a dog's logic, and it makes the loss feel both innocent and heartbreaking.
The feeling is bewilderment more than neat sadness. The world "tastes like a thrown stick," a strange, sensory image that suggests everything has become something to chase with no one there to throw it. In the second stanza the dog starts to run, then stops, expecting a tug on the collar that never comes. Even the "comforting rebuke" is missed, which shows how much the dog valued being kept in check by someone who cared.
The ending is the most painful part. The dog tries to say his name but can only howl. The mood is quiet, raw and tender, and it works because the dog never explains its grief, only shows it.