The Brain—is wider than the Sky— For—put them side by side—
The poem kicks off with a big bold statement, saying that the brain is wider than the sky.
We're going to go ahead and assume that the speaker doesn't mean this literally; it would have to be some kind of giant evil alien brain to be literally bigger than the sky.
So we figure the speaker is making some kind of metaphorical point with all this.
Line 2 sets us up to compare the two things with the rest of the stanza—we're going to put the brain and the sky side by side.
It seems like the speaker is going to prove to us just why she's right about this.
Oh, and before we move on, don't miss all the assonance in these first two lines. See all those repeated long I sounds?
Yeah, that's not an accident; it's a poetic device that helps the whole thing get into a sonic groove.
The one the other will contain With ease—and You—beside—
Ah, okay. So our speaker's definitely getting all metaphorical on us here.
She's saying that the brain is wider than the sky because the expanse of the imagination is infinite—it can imagine the whole sky and, well, anything else it wants. In fact, the brain can easily conceive of a billion skies and nothing can stop it.
On top of that, it can even conceive of us, the reader, right along with the sky.
(Is anybody else freaked out by this brain thinking about them? We're thinking about getting a restraining order. )
Real quick before we move on: did you notice that the second and fourth lines of this stanza rhyme? You've got "side" and "beside." That ABCB pattern goes through the whole poem. Check out "Form and Meter" for the deets on this rhyme scheme.