The line this question refers to comes in the third stanza of this famous poem, and describes the decision that the speaker made. Reflecting on the two almost identical paths, the speaker has to make a choice, but as he picks one path, he consoles himself with the thought that he can come back another day and take the other one, even though he swiftly realises that this is unlikely:
Oh, I kept the first for another day!
Yet knowing how way leads on to way,
I doubted if I should ever come back.
There is significant irony in these lines: what the speaker is doing is trying to convince himself that, although he has to make a choice at this moment, it will not be a choice with lasting consequences. He is trying to fool himself that the choice he makes on this pathway can be somehow unmade later on and he can come back and take the other path and see where that takes him. Of course, when the symbolic meaning of the poem is remembered, this is completely inaccurate. The meaning of the poem explores how we often have to make decisions in life and choose a path, and once those decisions are made, we are never able to go back and unmake them. This is why the speaker is so haunted by the other path he did not take, just as the majority of people stop and think about other choices that they could have made and where they would be today if they had made them.