Your question is really asking about symbolism and how it is used in this great poem. Robert Frost in his poetry is a master of symbolism, and uses every day situations or events to suggest much deeper meanings and ideas. Symbolism is defined as any object, character or action that has its literal meaning but also points towards a larger idea or concept. Thus, for example, a symbol of a dove both represents a dove, but it has also come to represent peace in general.
Applying symbolism to this poem then yields fascinating results. The surface meaning is the literal meaning or situation that the poem is describing. It is a poem about a man who stops by woods on a snowy evening and contemplates the scene before him, then carries on. However, much has been written about the symbolic meaning of this poem, and in particular what the woods symbolise. My own idea is that the woods represent death, or release from the world of work and responsibility. Note how attractive the poem makes the woods out to be:
The only other sound's the sweep
Of easy wind and downy flake.
The woods are lovely, dark and deep.
It appears that there is a massive internal conflict going on in the author about his desire to stay in these "lovely" woods, but then his recognition that he "has promises to keep." These promises could be said to symbolise the responsibilities of our world and work that we have to do. The repetition of the last line and the significance of the final word, "sleep," seems to support my conclusion, as sleep is a common symbol for death.
Thus, thinking symbolically, this poem is all about the speaker's desire to embrace the beauty of death, rest or freedom, but his recognition that he still has lots of work he must do before he can get to that rest.