This is a richly symbolic poem where things work on two levels - a literal level and a more symbolic level. This poem is usually used as an example of symbolism to help students explore symbolic meaning, or what lies beneath the surface. Therefore, it is important to try and work out or discern what are symbols in this poem to discover that Robert Frost is trying to say through his poem. There is much more to this poem than a literal pause in a wintry journey. The secret to working out symbolic meaning is to pay attention to clues the poet plants, such as repetition, emphasis, word associations, and mysterious images.
Bearing this in mind, examining the poem, an in particular the last stanza, we discover that this poem is actually about the desire for death or "rest" against the long days of hard work ahead. Consider the final stanza:
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.
Notice how the woods are described in an attractive way - they are "lovely" but also "dark" and "deep", however, in spite of the temptation the speaker feels to stay here and rest, to "sleep", he recognises that unfortunately, duty calls, for he has "promises to keep" and obligations to fulfil. The repetition of the final line really serves to underline the reluctance that the speaker has in continuing with his journey and the work of life - "sleep" is an attractive prospect, especially in such a place of beauty.
Thus the theme of the poem is to do with obligations and duty, and how we need to keep pressing on with our commitments, even though, at times, rest or death is a very attractive prospect.