The whole poem is metaphorical. The two roads that diverge are a metaphor for the possible life paths that a person can take. In much the same way that the author could not travel both roads, a person cannot take two life paths. And in much the same way that we might want to explore a particular option we have bypassed, later on our present journey takes us down roads that make it not possible to do so:
"I saved the first for another day
Yet knowing how way leads on to way
I doubted if I should ever come back."
There is imagery in stanza 1: "Two roads diverged in a yellow wood," and again in stanza 3 where he paints a picture of roads in the early morning littered with leaves "that no step had trodden black."
There is overstatement in stanza 4 when he says "ages and ages hence,” since the author could not be that old.
There is antithesis--the presentation of a contrasting idea to show a balance--when he says in lines 1 and 2 that two roads diverged but he is sorry he could not travel both. There is also antithesis in stanza 2 when he says the second path had perhaps the better claim as it was grassy and wanted wear, and then he comes later in the stanza to say that both paths had been worn "really about the same.”