The poem consists of four stanzas of four lines each. The rhyme scheme is well thought-out. The first, second and fourth lines of each stanza rhyme. The third line of each stanza rhymes with the first, second and fourth lines of the next stanza. The last stanza is an exception, all lines rhyme and the lines three and four are identical.
This rhyme scheme causes a very smooth transition between the stanzas. So the stanzas are connected metrically and we can expect that they are connected thematically, too. At first sight the poem seems to be very simple but if you look at it more closely there is a hidden meaning in it. There are no metaphors in this poem and not many technical features, either. There is one alliteration "dark and deep" in line 13. It could be considered that the narrator is wishing for his own death.
This is likely the reason why the last line is repeated twice. It gives the effect of sighing. The repetition also shows that this sentence is very important and the reader has to understand the meaning of this sentence to understand the poem. It is conspicuous that the narrator used much symbolism.
The idea of sleeping in the last two lines is an obvious symbol of death. In relation with the idea of sleeping the word "downy flake" in line 12 reminds of a soft pillow where you can sleep. In line 8 he talks about "the darkest evening of the year." You can associate darkness with evening and sleeping. So this symbol also supports the idea of sleeping.
The narrator is far away from the city and the city is a symbol for life. One of the opposites of life is death. The impression of silence, which is caused by lines 11 and 12, because the narrator is talking about very low noises, gives also the impression of death, because there are no other people or animals that make noises. In lines 6 he says that he stops between the woods and a frozen lake. In this case the woods are a symbol for life and the frozen lake is a symbol for death. He stops between them.
So he is considering if he should die or go on living. But then his little horse "gives his harness bells a shake to ask if there is some mistake." That is reminding him to come back and is reminding him of his duties. The lines 13 and 14 give the main statement of the poem again. "Lovely, dark and deep" these words are normally not associated. But for the narrator "dark and deep" is attractive to die. Line 14 "But I have promises to keep" gives again the impression of sighing and shows that this decision is hard to him.
The poem is about a person who stops with his horse by woods on a snowy evening and considers about dieing there. But he has "promises to keep" and so he goes on living.