by the snake's passage through it, like a comb parting hair.

A further metaphor, "Nature's people," is used to describe other animals—this, too, is a form of personification. Remember that personification is a type of metaphor, although of course many examples of metaphor are not also examples of personification.

Most people, when happening upon a snake in the grass, are startled. But note that in the first stanza, the speaker calls the snake a "fellow"; this is an innocuous term for a boy or a man. The speaker personifies the snake, making it seem less threatening.

In the second stanza, the speaker uses a simile to describe the snake. The snake is the comb dividing the grass (hair). The grass parts, and we can see the "spotted Shaft" (the snake). As the snake moves on, the grass closes "at your Feet / And opens further on -." The grass opens and closes as the snake makes its way through. It would be startling to see a snake move through your feet, but the image of the grass parting and closing seems almost peaceful.

In the third stanza, we learn that the speaker is male. And in the fourth stanza, he describes how he has encountered a snake multiple times before. He notes how he thought the snake was a whip. When he stooped to pick up the whip, he realized it was a snake as it moved away. Again, we have an image of something threatening but innocuous. A whip is dangerous but inanimate. The snake might be dangerous, but it moves away.

In the final stanza, the speaker says he's never met a snake (by himself or with others) without feeling some degree of fear. And in some cases, it is extreme fear, as he's chilled to the bone ("Zero at the Bone").

Dickinson mostly avoids any words that overtly call to mind phallic comparisons and/or references to the evil serpent in Biblical literature. That doesn't mean that these cannot be involved in reader interpretations. In the story of the Garden of Eden, the snake was friendly but actually something to be feared. The phallic/sexual innuendo suggests a similar dichotomy of conquest and pleasure via seduction. By avoiding overt allusion to these images, Dickinson might have been addressing sexuality or seduction in a covert way. On the other hand, this poem is about the beauty and power of nature: something to admire and fear.

Imagery is the use of words by an author that evoke a certain image in the mind of the reader. Common imagery is the use of similar images, or related images, throughout a piece of writing.

In this poem, nature imagery - references to the grass, the acre, etc. - are common. Also, references to the human being - fellow, barefoot, bone - are also common.

Checking out some of the scholarship eNotes provides on this poems validates ane exands upon Jamie’s insigtful response.  According to one critic, the poem is on the one hand realisitic depicting that snake and as such a detailed and imaginative description of an encounter with nature, but the poem is also about “transformation” in viewing nature, which creates “a more woman-centered religion that incorporates a reverence for the things of the earth.”  This view diminishes some of the violence and anxiety apparent in the poem, but affirms the system of symbolism that Jamie finds.

This is a sexual poem if you look at some of the symbols.  There is the "spotted shaft" and the snake that "divides the grass as a comb"... references to the sexual organs and the act of intercourse. 

However, the imagery is not positive.  The use of the "snake" itself has overtones of biblical evil and the snake is traditionally associated with secretive and vile acts.  Other descriptive words include "slither" and "scally" and I would argue out that the shaft (the penis) describe as "spotty" means that it is diseased in some way.