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.
The final couplet is one of the most famous instances of repetition in American poetry. 
 The speaker is exhausted and wishes he could fall asleep. His repetition thus has a weary quality. On the literal level he still has a long ways to go before he’s home. Metaphorically, “miles to go” suggests life; “sleep,” death. The metaphorical linkage between sleep and death is as old as the most ancient mythology, and echoes through some of the most famous passages in English literature; see e.g. Hamlet’s  “To die, to sleep.” 
 If we read the previous line (“promises to keep”) as the speaker yielding to his societal obligations, this couplet may indicate how difficult fulfilling those obligations will be–how hard the road ahead is. Tensions like these are part of why critic Lionel Trilling famously called Frost (against the opinion of most critics at the time) “a terrifying poet.” 

Stopping by Woods on a Snowy Evening