On the level of plot, the poem begins pretty simply. The speaker saw an "antlered buck and his doe," meaning a male deer and his mate, standing in an apple orchard earlier in the evening. Even these seemingly straightforward lines, however, are filled with rich symbolism.
The speaker begins by posing a question to the sky, which is white because it is a cloudy winter day. Winter is a season symbolically associated with death (think about it: this is the time of year when the ground freezes, plants are dormant, and many animals hibernate), and thus in calling the sky "white," the speaker links the snowy sky to death as well. By directly addressing the sky directly, the speaker also personifies it, implying that the sky is a figure watching "over" the earth. Altogether, this imagery thus establishes the omnipresence of death. That is, the white sky looking "over the hemlocks" symbolizes the presence of death constantly lurking in the background of the poem, and of life.
The speaker specifically positions the sky "over the hemlocks bowed with snow." Hemlocks are a type of evergreen trees, meaning they remain green throughout the year. The color green is associated with spring and new life. Yet these hemlocks here are "bowed with snow." On a literal level, this just means that their branches are loaded down with snow. Symbolically, though, this image of green trees sagging under the weight of winter snow again implies that supremacy of death, which rules over all of nature.
In line 2, the speaker refers to "the beginning of evening," revealing that time has passed since she first saw the animals, but not that much time; after all, it is still the same day. This specific language also adds to the sense of foreboding in the poem: evening is associated with darkness, which is usually associated with despair, sorrow, and death. Therefore, the "beginning of evening" represents the transition from a period of light—daytime—to a period of darkness. As it moves on, the poem itself progresses further into darkness—symbolically, into sorrow and death.
The image of the "antlered buck and his doe," however, contrasts with the foreboding setting of the poem. An antlered buck, or male deer, is one that has reached sexual maturity. Moreover, this buck is paired with a "doe." The pairing represents the vigor and hope of life, and the potential of new life. Their presence in the "apple-orchard" also suggests that they are there in search of sustenance to help them survive the winter.
The first three lines thus set up a sharp symbolic contrast between death, as represented by the white sky, the "hemlocks bowed with snow," and "the beginning of evening," and life, as represented by the "antlered buck and his doe" and the "apple-orchard."
In terms of structure, "The Buck in the Snow" is not written in any particular form or meter. Rather, like many of Millay's poems, the poem is written in free verse (albeit with frequent rhyme). The lack of structure and pattern in the poem suits the content of the poem, which addresses the unpredictable and unstructured nature of death.