Amy's poem watches a sheepdog on duty and lets us see the world the way he does. The opening lines, "Hours pass. / The watchdog waits," set a slow, patient scene. He is "steady as a rock," present but silent, wanting to be seen but not heard, as if his mere presence is meant to be warning enough.
The turn comes with the intruder. To us, it's only a cat, harmless and a little funny. To the dog, it is "a villain," even "a usurper," a word that suggests someone trying to steal a throne. That gap between our view and his shows how seriously he takes his job.
The second stanza repeats "Hours pass," but now the poem turns to what lies behind his patience. Time means nothing to him, and he would sit and stare forever as long as the sheep are safe. The mood is calm and steady, with a quiet admiration for loyalty that asks for nothing in return.