"The Road Not Taken" by Robert Frost follows a long tradition of poems and speeches based on the idea of "two roads." The classical model with which Frost was probably acquainted was a speech by Prodicus (reported by Plato) known as the "Choice of Heracles" describing a smooth easy path sloping downwards gently to vice contrasted with a long, steep, rougher upward path leading to virtue. Although in Frost's poem the roads do not differ so dramatically, there is still a sense that the rougher and less traveled road is somehow preferable.
The two roads generally symbolize life choices. Any time we make a choice in life, we are excluding other choices. If, for example, you choose to go to one college, this means not going to another college, and if you choose one major, it means not choosing another major. In commenting that he is not likely to return to the specific fork in the road, Frost is suggesting that although we may be capable of revisiting our choices, we often do not.