The poem itself is so small, both literally and in terms of its content. It's about an itsy-bitsy spider, a flower, and a moth. Thanks to the title, though, we know immediately that we are talking about some serious philosophy. The poem's title is a direct nod to a classical argument for the existence of God. It goes all the way back to Plato, but Frost is probably referencing a guy named William Paley. You can check out his argument in detail here, but the long and short of it goes like this: in nature, the remarkable complexity and careful, purposeful design we see can only be the work of an intelligent creator.
As we see throughout Frost's poem, though, we learn that this proof isn't all that helpful because it leaves us with a God who must have designed and controlled everything, even really awful, appalling things—like moth murder. It's also worth noting here that the poem itself is a remarkably designed sonnet. (Check out "Form and Meter" for all the details.) No doubt Frost tightened down his poem to such a precise sonnet to honor his own title. In that way, he's the great Designer behind this poem—props to him.