Welcome to the silly, comforting poem. (1.1)
Well now this is unusual. Our speaker's being so upfront: "Hey this is a poem; welcome." We also get a sort of vague definition of poetry: something silly and comforting. Which kind of makes sense. We mean, from a certain perspective, spending so much time with a few words on a page is kind of silly. As for the "comforting" part, well, we guess our speaker believes that poetry can provide a sort of consolation or encouragement. But we'll have to keep reading to find out if that holds water.
It is not the sunrise (1.2)
So now that we've been introduced to the poem, our speaker begins defining what the poem is not. Right at the beginning, our speaker wants us to understand that a poem is not the same as the things it describes. Although this might make us scratch our heads, it actually kind of opens up the possibilities. Once we quit thinking of poems as simply being pieces of the world, we can recognize that maybe they're a lot weirder than that. They refer to things in the world, but they kind of exist on their own little plane.
it is not the mockingbird who, in his own cadence,will go on sizzling and clappingfrom the branches of the catalpa that are thick with blossoms (1.8-10)
In this description, the speaker shows poetry's knack for conveying amazing images. Even if the poem-world is not the same thing as the real world, it still refers to it and can impact us through those references. Our speaker might remind us, though, that the transformative awe we experience when reading that description is an entirely different experience from the transformative awe we might feel if we saw the scene in our backyard.