What is Hopkins saying about God?

Hopkins is saying that God made a world that is beautiful because it is full of variety and that we should praise him for that. Something "pied" has different splotches or patches of colors. As his title indicates, Hopkins celebrates God for making so much of the world "pied." He mentions all sorts of variety, such as skies that are streaked with two colors, a brindled (color-streaked) cow, and the contrasting colors on a trout. All of these are the "dappled" or spotted things that Hopkins appreciates in God's creation.
Basically, Hopkins is glad our world was not created in one color or with a repetitive sameness. His speaker enjoys being surprised by parts of nature that are original. He is grateful for what is "original, spare, strange." He draws an implicit contrast between what is created in a factory—items that are completely uniform—and what God creates, which consists of endless variation.