"In Praise of Creation" starts with some small, specific sparks of the creation it praises. The speaker begins with a list of beings that are paradoxically united by their difference:
That one bird, one star,
The one flash of the tiger’s eye
The anaphora here, shaping each clause similarly so that each of these distinct beings is that "one", brings together all these distinct creatures under the umbrella of creation, but it also sets them apart. Everything the speaker lists is one among a zillion: think of all the birds in the world, all the stars in the sky, and all the uncountable times a tiger's eye might flash over the course of its life. And yet, here, each of these beings is singled out as unique, with its own identity.
They "[p]urely assert what they are": they speak of their own individuality simply by being. By placing the solid "bird" and "star" next to the fleeting "flash of the tiger's eye," the speaker also suggests that all of these things aren't just things, but moments. Birds, stars, and tiger-eyes are all part of nature, but also of time.
Think back to the title of the poem. These introductory images are meant to be "In Praise of Creation"—and the word "creation" fits right in with this idea of creatures as solid and specific, and as moments in time. "Creation" can mean "everything in the whole world," but also "the process of making things." The speaker will be dealing with both of these kinds of creation: what's solidly here, and what's always in the process of being made.
These distinct flashes of existence, the speaker says, "testify," provide proof. But what might they testify to?