The entirety of the line you are referring to in Robert Frost's poem "The Road Not Taken is "In leaves no step had trodden black."
This line is an apt and pithy description of the road the speaker is taking. The speaker of the poem has come across two diverging paths in a forest area. He wonders which path he should take, and he is sorry he cannot split himself in two so he can travel through both. Both have been well-traveled, and he can tell from the evidence of the "trodden black." It refers to the leaves on the ground, which have been stepped on by many people. The dirt on their shoes and the slow decomposition of the leaves have turned them darker. However, the speaker decides to choose the path that ends up being "less traveled," which means less people have walked through it. He wants to come back and choose the other path the next time, so he will have experienced both, but a part of him doubts that his future travels will bring him back to this particular place.
The blackness of the leaves also contrasts with the initial "yellow" color that Frost invokes in the beginning line. The wood he is traveling through is described as "yellow." Because these are the only two colors mentioned directly in the poem, they stand out as an interesting juxtaposition. Frost conjures yellow, a bright and sunny color that seems to bring an image of a beautiful Autumn day. Then he conjures black, dark and harsh against the yellowness of the wood's canopy. The intense variation lends to the feeling of the poem: a bit lonely, bittersweet, and nostalgic. It seems to capture the complex feelings of limited decision making, wanderlust, adventure, and the metaphor of the traveler's dilemma.