Before answering this question, it helps to check out the final stanza in Robert Frost's "Stopping by Woods on a Snowy Evening":
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. (13-16)
Many scholars, critics, and readers have proposed that this seemingly simple poem is actually a depiction of an individual contemplating death and/or suicide. One of the factors that points to such an interpretation is this final stanza, especially the first line: "The woods are lovely, dark and deep." The darkness Frost references, especially when combined with the reference to sleep, seems to symbolize a kind of death. Moreover, the idea that the speaker of the poem eschews such darkness based on the fact he has "promises to keep" suggests that he has some control in choosing whether or not to give into the referenced darkness. This fact could be seen as a suggestion of suicide and, once that idea comes into one's head, it becomes difficult to read the poem and its suggestion of darkness as anything else but a contemplation of death.
That's not to say that this is the only way to read the poem. The meaning of darkness remains fairly ambiguous and, while it certainly could refer to death, it could just as easily refer to something else entirely. Some readers, for instance, point out that the speaker could be talking about rest in general, and so one might not need to wrestle with death at all. In short, there are many readings of this poem, and many interpretations of its "darkness." That said, the idea that darkness suggests death or suicide has proven to be one of the poem's most long-lasting interpretations.