Two roads diverged in a yellow wood (1)
From this moment, we know that the speaker is going to have to make a choice and walk down one of these roads, leaving the other one behind. This line could represent many different choices in the speaker's life, but he's using a metaphor rather than describing them specifically. This lets us readers think of choices that have been significant in our own lives when we read this poem.
Then took the other, as just as fair (6)
After looking down one path for a long time, our speaker takes the other one. With a choice that is shown to be significant by the end of the poem, this is impulsive and risky. We think that the speaker was a little bored with the path that he was going to take, and wanted to shake things up a little.
Oh, I kept the first for another day! (13)
Our speaker is no longer so sure about his choice. What he thought made the road he took better – that it was less traveled – now seems untrue, and he's having a hard time reminding himself why he decided to go down that path anyways. But he comforts himself by thinking he can just try the first path on another day. We've all done things like this – like, we can't decide to go on vacation to the beach or the mountains. We choose one and figure we'll get to the other sooner or later.