The only real action that happens in Robert Frost's "The Road Not Taken" is a choice: the speaker of the poem is at a crossroads and must choose which path to take. It becomes clear that there is no bad choice, but he does have some sense of regret about the path he might have taken.
In terms of how they look, the two paths are quite similar. As he stands at the crossroads and looks down both paths, he at first seems to prefer one over the other; however, in the end he determines that one road is "just as fair" as the other and the people who have traveled these two roads "had worn them really about the same." On the morning he makes the choice between the two roads that "equally lay," he travels one and
...kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.
He knows, as we all do, that he will probably never come back to this place again to explore the road he did not choose. He is not angry or bitter, but he is is a realist and he knows his life will probably not lead him back here again.
The other stanzas are a discussion about the two roads and making a choice; the final stanza reveals how the speaker feels about the decision he made:
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.
The speaker realizes, of course, that by traveling one path (following certain goals and dreams for his life), he necessarily gave up what might have been if he had made different choices. We all do this. For example, choosing one college to attend is great and necessary, but it eliminates everything that might have been at another college. Choosing one field of study to major in necessarily restricts the study of another subject area. Choices must be made, but even when both choices are acceptable and good, there is still a sense of wistfulness and/or regret. This is often expressed as "what if?"
In this poem we know the speaker had to be thinking this way about the choice he did not make, for the title is "The Road Not Taken" rather than "The Road I Took."