This phrase appears in the third stanza of the poem:
And both that morning equally lay
In leaves no step had trodden black.
The first line of this stanza lets readers know that the speaker is at the fork in the road in the morning. We also know that the season is autumn, because the first line of the poem tells us that the speaker is traveling through a "yellow wood." The leaves have turned from green to yellow (and likely various other shades). Over the course of the night, the trees continue to drop their leaves on the ground, and the paths have been covered with golden yellow leaves. As people walk over those leaves, the leaves are pressed into the dirt and mud of the path. That will turn the leaves from shades of yellow to browns and blacks. As more and more people travel the road, their feet/shoes pick up some of that dirt, mud, and crushed leaves. That debris will then get deposited on top of more leaves. Eventually, the leafy yellow path will be changed into a leafy, muddy, blackened path. The speaker in the lines above is simply letting readers know that he is at the fork in the road before enough people have walked on the path to dirty it up.