This poem uses imagery, language that conveys sensory information, in order to help us understand how solitary the speaker is, and how dark and beautiful the woods are. The second stanza is most concerned with visual imagery: things we can see. There is no "farmhouse near" and the speaker has stopped to rest "Between the woods and frozen lake" on this, the "darkest evening of the year." It is easy for us to visualize his location with the help of these images. The third stanza is most concerned with auditory imagery: things we can hear. We can imagine the sound when the horse "gives his harness bells a shake" as well as the near-silence of soft-blowing breeze in "the sweep of easy wind and downy flake." The words, "downy flake," work as a visual image too: we can imagine the kind of snow that is fluffy and drifts slowly to the ground.
Some readers also interpret "sleep" as a metaphor for death in the final two lines which repeat, "And miles to go before I sleep." In this reading, the speaker's solitary journey through the woods—the "miles" he goes—becomes a metaphor for life, for all the work we must do before we can go to our final rest, so to speak.
In addition, I agree with the other commenter who discussed the light personification of the horse in the second and third stanza. To attribute the abilities to "think it queer" to stop in such a solitary spot and to shake his harness bells "to ask if there is some mistake" qualifies as personification, which is when the poet gives human characteristics or abilities to something nonhuman.