One popular approach to interpreting creative works is that there is not necessarily a right or proper answer. When a piece of creative fiction is written, that piece becomes removed from the intent of its creator, and readers will do what they want with it. Poetry is a rather monstrous undertaking in this regard because the meaning of a well-written poem hinges rather pointedly on the construction of the poem, including diction, rhythm, and form. A hundred different people might interpret the same poem in a hundred different ways, all of which might be unique yet still well in line with the actual form of the poem.
Frost's work continues to appeal because it skillfully invokes ambiguous feelings. When you read "The Road Not Taken", you probably get the distinct impression that it has to do with the impact a single small decision has on events which follow that decision. This notion implicitly imparts a sense of importance to even the smallest moment in time.
More specific interpretations of this poem, however, vary remarkably. this line seems to leave the most powerful impact on many people:
I took the one less traveled by,
Here's where an interesting set of problems of interpreting poetry rears its ugly head. Authors of poetry are imperfect, speakers in poems are unreliable, and language is an imperfect vessel. This previous sentence, for example, might seem at first glance to suggest that the speaker really did take the path less traveled by. But if you take it in the context of these previous lines, it becomes less clear that that is the only possible interpretation:
Though as for that the passing there
Had worn them really about the same,
And both that morning equally lay
In leaves no step had trodden black.
Considering these lines, a different interpretation might suggest that the speaker in the poem is embellishing when he later claims that he took the road less traveled by--and this becomes ironic when you consider that the most obvious message someone might drag out of this poem is that small decisions are important.
If you take the speaker to be embellishing here, you might assume the speaker is embellishing everywhere in the poem. This might get you thinking that the speaker feels a deeply situated regret for having chosen the "path" that he did and that his telling of this story in this way is more a lie to himself than is meant to be a lie to the reader. Alternately, you might take a less complicated approach to interpreting this conflict and determine that this collision of language is an honest mistake on the part of the author (unlikely), or that the speaker is simply failing to say what he means in a quite correct way (very possible).
One thing people often forget when reading poetry is that the speaker in a poem is almost always imperfect, especially if the speaker is a human being. Speakers in poems can and do lie, misremember, and misspeak, and in some pieces of fiction the narrator or speaker might tell a story in such a straightforward way that it is almost but not quite perfect--just like a well-told lie in life. The beauty in those poems is in following that very tiny mistake all the way down to a profound complication in the speaker's character.
Even with this one poem, you could probably write a very long article analyzing all the possible interpretations thereof or justifying a very particular interpretation you might favor. I won't go into trying to present all the possible interpretations of this piece. The point of this answer is to remind you of two things you might be forgetting: that people write poems and that speakers in poems are people. If you always trust explicitly what's written on the page, you will have a terrible time interpreting more challenging poems you're likely to encounter in academics.
