The speaker comes to a "big dog" and lays his head in its lap, like an offering. The head is "full of breathtakingly beautiful images" of the dog itself, so what he brings is his whole mind, crowded with admiration. It's a strangely tender picture of devotion, with the human giving himself over completely to an animal.
The feeling is one of release and calm. Once the head is in the lap, the speaker "breathe[s] more quietly," as if the act of offering has settled something in him. The short, simple sentences, each beginning with "Now," slow everything down and make the moment feel almost like a ritual. The dog's pat is a small reward, and "Now I sign" suggests a contented sigh, though "sign" may be a typo or a deliberate twist on it. The last lines are gently funny and a little sad: he knows he'll soon "be a man again," as though for these few moments he has been something simpler, and humbler, than human.