A young retriever spends these six short verses chasing things that can't be caught. Butterfly shadows haunt him even in his sleep, and when he's awake his nose, usually so reliable, goes "wild" because the shadow has no smell. His legs seem to run ahead of the rest of him, and when a shadow shrinks and disappears on the lawn, he digs for it as if it had been buried.
The speaker mostly watches, with fond amusement and no mockery. The dog's logic is simple: if something moves, it is worth pursuing. The last two verses carry him into a creek, where he keeps his eyes open underwater and finds stones alive with moving shadows. The final word, "Delicious!", is the most striking moment, since it seems to put the puppy's own delight into words. The mood is playful, quick, and full of wonder, suggesting that the world looks richer when you give it your full, curious attention.