"The Summer Day" begins with a question that human beings have grappled with essentially forever: "Who made the world?" Whether the speaker means the whole universe or just Earth, that's a big question to ask! Rather than try to answer it, however, the speaker immediately asks two more questions. These are a little more specific than the first, but they're still unwieldy, difficult, and related to creation.
These three questions can be called rhetorical, since they aren't really looking for answers. It's not even clear whom the speaker is talking to; the questions could be addressed to nature, an unknown deity, the speaker's inner self, readers of the poem, or no one in particular!
These questions indicate the awe that the speaker feels when thinking about nature. For this speaker, the simple existence of the natural world and its many kinds of inhabitants (from the swan to the black bear to the grasshopper) is a miracle. The speaker appears to be a curious, attentive, imaginative person—one who is perhaps more interested in asking questions than insisting on firm answers.
These opening lines are written in free verse; in other words, they don't follow a regular meter or rhyme scheme. The rest of the poem will follow this pattern, too, using lines of various lengths and rhythms. The flexibility of the verse allows for a wide range of expression and gives the poem an intimate, conversational feeling.
The title indicates that the poem takes place on a summer day, perhaps in a place where there are swans and black bears (as readers will see, there are definitely grasshoppers!). Notice that there's a subtle difference between calling the poem "The Summer Day" and "A Summer Day." A summer day might seem unremarkable—merely one of many days in the season—but the summer day is something special.
This slight elevation raises the spiritual, philosophical, and personal stakes of the poem and gives it an almost allegorical quality. This is the day when a special thing happened (or else, every day is the day to start making the most of your "wild and precious life").