Section 1 Summary

Get out the microscope, because we’re going through this poem line-by-line.

Welcome to the silly, comforting poem.

Why thank you, speaker. That's awfully nice of you to welcome us. And we're mighty glad this poem will be both silly and comfortable. We could use a bit more of that in our lives.
It gives us a warm fuzzy feeling to be greeted like this. Most poems don't stop to talk to us.
And she acknowledges that poems are kind of odd, silly things. We're glad we're not the only ones who think so.
The word "comforting" seems to suggest that poems serve some emotional purpose—or at least this one does.

It is not the sunrise,
which is a red rinse,
which is flaring all over the eastern sky;

Oh, okay. Can't really argue with that.
Our speaker kindly informs us that this poem is not the sunrise, thank you very much.
Still, even though it isn't the sunrise, she takes some time to tell us more about that sunrise, using a metaphor when she calls it a "red rinse."
The word "rinse" makes it sound like the sunrise is cleansing in some way. Is this connected to the "comforting" from the first line?
Also, using the word "flaring" makes us think of a flare. And flares are used as signals, to call our attention to something. Of course it's also the title of the poem.
What can we take away from these lines? Well, even though our speaker is insisting that the poem is not the sunset, we can tell she's interested in natural things.

it is not the rain falling out of the purse of God;

The poem is also not rain sent down by God. Well, that's good to know.
But why does she keep telling us what the poem is not? And what's with all the nature?
So at first we thought: rain falling out of a purse? Why would rain be in a purse? And why would God have a purse?
But now we're thinking about the fact that a purse might have coins in it. And we could see how raindrops might look like a bunch of tiny silver coins falling from the sky. (Plus what use would God have for money? We figure He'd be more likely to save up something useful and beneficial like rain.)
It sounds like the speaker wants us to acknowledge that this poem isn't something fancy or grandiose from nature. It's just, you know, a poem.

it is not the blue helmet of the sky afterward,
or the trees, or the beetle burrowing into the earth;

We think we're starting to get it now.
Our speaker is telling us that the poem is not the same thing as the things it describes.
What we mean is: the word "sunrise" is not an actual sunrise. It's just something that represents a sunrise in language. And the word "beetle" is not going to crawl off the page and burrow into your front lawn. The words are like references to the things, but they are not the things themselves.
We think that maybe she's talking about the complex relationship between words (or symbols) and the things they represent, but without sounding all abstract and stuffy.
And of course, she's sticking with her natural imagery. We're betting that will become more and more important as the poem picks up speed.

it is not the mockingbird who, in his own cadence,
will go on sizzling and clapping

Okay, one last image: the poem is not the mockingbird, which is singing and flapping about.
The way she describes the mockingbird as "sizzling and clapping" is a bit strange.
"Sizzling" kind of brings up the idea of singing (they sound pretty similar), but it also has this whole other sense that it carries with it. It makes the mockingbird seem vibrant, blazing with life.
"Clapping" could be, in the more literal sense, the sound of the bird's wings. It also carries with it all the associations we have with clapping, like applause.
We clap when we like something, when we want to celebrate it or acknowledge how good it is.
So in this description the mockingbird (like our speaker) is kind of singing about or celebrating the world. Which brings us to…
… Okay, brace yourself. We just had another breakthrough: A mockingbird is a bird that imitates the songs of other birds. And our speaker is writing a poem (description) that imitates the world. Nifty, right?

from the branches of the catalpa that are thick with blossoms,
that are billowing and shining,
that are shaking in the wind.

The mockingbird is in a catalpa (a kind of flowering tree), whose branches have lots of blossoms. The branches and blossoms are shining in the sunlight and moving with the wind.
Here we think it becomes especially clear that this poem is our speaker's own celebration, her own song. This description is lush and beautiful. It feels like a celebration of the natural world (specifically the mockingbird and the catalpa tree) that our speaker encounters on this early morning.
Though we must say that our speaker's acknowledgement of the separation between words and the things they describe makes her celebration especially strange to think about: It's a celebration of the world through descriptions of the world that are actually separate from the world. Yeah, we know. Our minds are blown, too.