The Brain—is wider than the Sky— For—put them side by side— The one the other will contain (1-3)
The first stanza is a real testament to the speaker's belief in the power of the human brain.
The brain can contain the sky? Wow. The sky seems pretty endless and uncontainable to us, especially if you count all of space along with that. Could the speaker be saying that the brain can imagine infinities beyond even the infinities we can see with the naked eye?
The Brain is deeper than the sea— For—hold them—Blue to Blue— The one the other will absorb— (5-7)
Although modern science tells us that the sea is actually a whole lot smaller than the sky, the second stanza seems to be delving into the same idea of infinity that the first stanza did. The "Blue to Blue" thing even reminds us of the blue of the sky and connects the stanzas in our minds. This time the speaker seems to be talking about the brain's amazing ability to absorb information, which she believes has no limits at all.
The Brain is just the weight of God— For—Heft them—Pound for Pound— And they will differ—if they do— As Syllable from Sound—(9-12)
The final quatrain is probably the speaker's biggest declaration of the power of the brain. She compares it the G.O.D., who for many people represents the biggest infinity there is. The speaker goes so far as to say that the human brain and God are on the same level. And when she says that if they differ at all, it's "As Syllable from Sound," she seems to be making the distinction that the mind of humans is shaped by intellect, while God is raw, uncontrolled nature. To some this statement might seem pretty blasphemous, but to the speaker it's a declaration of the incredible intellectual powers of the human brain.