OK, you got us – only one thing actually happens in this poem: the poet sees a bolt of lightning and reflects on how the bolt is like a fork. This leads her into an imaginative meditation on this piece of awe-inspiring cutlery: if the lightning is a fork, it falls from some mysterious table, dropped accidentally by some mysterious hand, in some mysterious house. We can't ever see who's doing the dropping, or where they live, but the glimpse of the falling "fork," the poem concludes, reveals to us (poor, ignorant humans that we are) the fact that these greater powers are out there.
The Lightning is a yellow Fork
From Tables in the sky
By inadvertent fingers dropt
The awful Cutlery
Of mansions never quite disclosed
And never quite concealed
Finally, in stanza 2 we get to the heart of the poem: the speaker proposes that this fallen "Fork" slipped down from a big, mysterious house up in the sky, which is neither seen nor unseen. "[N]ever quite disclosed/And never quite concealed" suggests that we're always aware that it's there, but we can't see it.
What's she referring to? Well, your guess is as good as ours – but it seems like Heaven, or the heavens, or wherever God lives. The word "mansions" suggests to us that it's a vast, possibly unending series of mysterious dwelling places that house an equally mysterious God.
The Apparatus of the Dark
Here's the kicker: this flash of lightning, the fork that accidentally falls from the divine table, is just part of the "Apparatus," or the framework of the heavens.
Apparently, we're only allowed to see this framework, but not the being that built it, or lives in it. The true nature of this power is still "Dark," unknowable, and mystifying.
To ignorance revealed.
Hey, who are you calling ignorant? That's right – we humans are. And, as noted in line 7, only part of the picture is "revealed" to us. Things like lightning and other phenomena show us that there is a higher power at work, but we're not privileged with any more details beyond that.