One of the central themes of the poem is the beauty of nature. To see this theme in the text, some of the more descriptive lines are useful
My little horse must think it queer
To stop without a farmhouse near
Between the woods and frozen lake
The darkest evening of the year.
(Frost, "Stopping by Woods on a Snowy Evening," eNotes eText)
This sets the immediate tone; the narrator is in winter, watching the woods "fill up with snow," and enjoying the silent nature scene. Frost uses simple words that work well together, drawing a mental image of a frozen lake surrounded by dark woods, and the snowfall. Although it is "the darkest evening of the year," there must be some moonlight or other ambient light for the narrator to see the woods; starlight wouldn't penetrate snow-clouds, so the image is of moonlit woods with "easy wind and downy flake" flowing through the trees.
The narrator sums up his feelings for nature with the line, "The woods are lovely, dark and deep," showing his desire to commune with nature. Although he can't take the time to enjoy the woods further, his memories and the mental picture painted by his descriptions are enough to keep him going as he travels "miles" to keep his "promises."