Whose woods these are I think I know.
His house is in the village though;
He will not see me stopping here
To watch his woods fill up with snow.
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.
He gives his harness bells a shake
To ask if there is some mistake.
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep,
But I have promises to keep,
And miles to go before I sleep,
And miles to go before I sleep.
Here the woods may function as a metaphor for death. Our narrator is world-weary; the rest that death could provide would be “lovely, dark, and deep.” Lulled into the woods, he’s effectively forsaken his sense of prudence and self-preservation. 
 In other words, this may be a death fantasy that the speaker entertains before yielding to his everyday obligations (“promises to keep”). He is excited by the possibility of the world without his life, or human life wholly, a tantalizing, impossible vision of the outlines of a stranger universe. It seems to promise everything new, because it is so distant, but it is an unstable, brief vision which obscures reality. Talk about seeing the forest through the trees! See Frost’s  “Birches”  for the development of a similar idea, also linked with an image of trees: 
 
 I’d like to get away from earth awhile, And then come back to it and begin over…. 
 I’d like to go by climbing a birch tree And climb black branches up a snow-white trunk 
 Toward  heaven, till the tree could bear no more, But dipped its top and set me down again. 
 
 Compare also one of Frost’s earliest poems,  “Into My Own,”  in which “dark trees” represent some form of dark and primal knowledge, which the speaker wishes would extend to “the edge of doom.” Frost returns to a similar setting and theme in  “Come In.” 
 Frost may even be tracing a link between the contemplation of death and the perception of sublime beauty. (See his contemporary Wallace Stevens’s famous phrase:  “Death is the mother of beauty.” ) 
 
 However, we don’t  have  to read the woods as a death symbol. The part of the speaker that’s wild may simply want to be free, to live in the lovely (natural and organic), dark (primitive, dense with life), and deep (rich in true meanings and morals). Death is arguably none of these things, but rather nothingness itself.
Stopping by Woods on a Snowy Evening