This whole poem is a kind of cool thought exercise that asks us to create an imaginary setting that ends up enacting the speaker's thoughts. Huh? Basically, it's a mind-bender that lets you wander around in Emily Dickinson's brain for a bit. First, we imagine a bolt of lightning to be a golden fork, which falls down to earth from a big dining table up somewhere in the sky. Then we begin to imagine the grand banquet hall it must have fallen from, hidden by the clouds, and framed up there by stars.
If the lightning bolt that looks so huge to us is just a dinner fork from the house up there in the heavens, how enormous must that house be? As the poem suggests, the "mansions" that go with lightning-bolt cutlery are so vast that they can only be half-imagined. They're "never quite disclosed" and "never quite concealed"; in other words, we only catch glimpses of them (like the lightning); we can't really wrap our minds around them.
The poem summons vague, fascinating images of this imaginary castle in the air, images that are veiled but highly suggestive. This process of imagining but not really knowing what to imagine acts out the speaker's attempts to figure out what's up there and her realization that we can never really know or understand the nature of the Universe and its higher power.