Stanza 2 Summary

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

When I walk
I part the air

The speaker explains what happens when he walks. So this is not the most action-packed line in the poem.
But hey, it's fascinating. The speaker is meditating on his ideas of existence—like what it feels like to be alive and thinking in the world—and he says he parts the air.
The invisibility of air and the image of the speaker's body parting that invisibility increase the interplay between the speaker and his location.
The wha…? Here's what we mean. This guy's so sensitive to his presence in the world that he imagines himself "parting the air" as if there are invisible curtains that he is walking through every time he moves.
Did you notice how the two lines are almost identical in length? And how the two phrases are neatly broken in half between two lines? Just like the speaker is parting the air when he walks, the lines are split into two equal parts.
That, folks, is how form imitates content. Just another reason to keep moving through this poem and see what's next.

and always
the air moves in

The speaker is sort of in to repeating himself. Just like he repeats "I am", he repeats saying "This is / always the case." In these lines, the air always moves in.
In other words, it's ongoing, never-ending, inescapable and, for the speaker, a constant part of his existence in the world.
There's a cool image here, too. At the beginning of the poem, the speaker uses words like "absence" and "missing," both of which connote stillness, but here is the image of air moving.
How does air move? He could mean like the wind, but it's more like an empty, invisible substance reclaiming its space in the world. Cool, right? Air moving is the idea of something invisible in motion and that's how our speaker feels, as an invisible man on the move.

to fill the spaces
where my body's been.

The air filling spaces? How is that possible? Sounds like our speaker has things backwards.
But that's what's so cool about this poem: by now, the speaker has mismatched presence and absence so that an invisible thing like air can "fill the spaces."
Just like the speaker is what's missing, as soon as he moves, the air he parted must rush in (think of trying to dig a hole in sand, and watching the sand slide back into the hole—annoying!) to fill where his body was.