From the opening line of "The Lightning is a yellow Fork," we get the feeling that the speaker is talking around something rather than talking about it. Why? Well, sometimes you're just so bowled over by something that you can't talk about it directly. The "something" here that the speaker can't quite express is not just the lightning itself, but what the lightning stands for: a sense of the great beyond. The lightning is just the tip of the iceberg, if you'll excuse our mixing of natural metaphors. The huge, dropped "fork" of its striking bolt is just a hint of what else is up there in the cosmos. Dickinson's poem uses metaphorical language because it's the only way she can talk through these awe-inspiring, mind-boggling ideas.
The speaker's use of metaphor in "The Lightning is a yellow Fork" takes the place of descriptive language. The speaker's inability to clearly depict the lightning in descriptive terms demonstrates her awe and incomprehension of this phenomenon.
The speaker of "The Lightning is a yellow Fork" definitely believes in something – the question is, what? We're not sure, and neither is she. However, not understanding something doesn't mean you can't believe in it. In fact, we often believe most passionately in things we can't explain. (Hello, Santa Claus?) The very idea of God has always had incomprehensibility written into it. In this poem the speaker attempts to communicate her sense of cosmic truth by showing us the ways in which nature's wonders (in this case, lightning) demonstrate the presence of a greater scheme of things. We can speculate about it, but we can never really understand it. It's these glimpses of that presence that reassure the speaker that her faith is in the right place, even if she can't explain why.
The disorienting contrast of natural phenomena with domestic images, and the idea of simultaneous revelation and concealment, both contribute to a view of religious belief that requires faith but accepts that true knowledge or understanding is impossible.
Weirdly enough, the "natural world" in "The Lightning is a yellow Fork" is mostly communicated in terms of the opposite of nature – domesticity. Dickinson craftily describes a natural phenomenon – the lightning – in a strikingly (no pun intended) homely and almost comforting way. Her mash-up of a wild, powerful natural event and an imagined domestic setting draws a connection between the workings of the natural world and the workings of our human one. The poem also goes on to demonstrate the idea that natural elements are all part of the same vast "Apparatus" that makes up the cosmos, and that nature, like us, is subject to higher powers.
This depiction of a natural phenomenon using everyday domestic imagery implies that the world we live in is part of a bigger scheme. The comparison of a lightning bolt to a dropped fork demonstrates that even nature is under the control of a higher power.
Living as we do in the age of the Internet, where it seems like all the world's knowledge is at our fingertips, it may be hard to put ourselves in Emily Dickinson's 19th-century shoes. Our technologies of information storage and retrieval would have been totally mind-blowing to someone of Dickinson's time. But even though we know a whole lot more about the world now than we did then, we still don't know everything – and we never will.
"The Lightning is a yellow Fork" is about this fundamental problem of human knowledge: the more we know, the more there is to learn. Dickinson's poem expresses an understanding of the fact that there will always be some things that are beyond our understanding. It seems the only thing we can know for sure is the fact that we don't actually know anything for sure; the revelation of the "Apparatus" or framework of the universe only serves to remind us of how little we truly understand.
The speaker gives up on her whimsical extended metaphor of a house in the second stanza, which reflects the impossibility of understanding the workings of the universe. This failed metaphor shows a kind of frustration with the limitations of human understanding.