When the narrator, who is walking through the woods, comes to a fork in the road, he ponders which way to go. He decides on the path that is "grassy and wanted wear," but also notes that each path is about equally worn.
The lines in question mean that at this point in the morning, nobody has yet travelled on either path where the road forks, because otherwise the leaves on one path or both would be stepped on ("trodden") and have turned darker ("black") from people walking on them. Each path, in other words, has a virginal quality. Whichever path the narrator takes, he will not be following in somebody's footsteps. He has to make his own decision.
As he choses the second path, he says he keeps the first path in mind for "another day." In other words, he rationalizes his difficult choice by saying he will sometime in the future see where the other path leads. But even as he thinks this, he already knows that it is unlikely, once embarked in one direction, that he will then head in another.