What do these lines mean: "And both that morning equally lay / In leaves no step had trodden black"?

These lines mean that, of the two roads the narrator is considering, neither one is more or less traveled than the other. At that moment, "that morning," when he is trying to decide which one to take, the roads have been "equally" traveled, and he can tell because no one has walked on the leaves to turn them dark with mud or dirt.

Each road symbolizes a choice; the narrator has two choices. The lines, "And both that morning equally lay / In leaves no step had trodden black," repeat the idea -- stated earlier in the poem -- that the roads, though they look a bit different from one another (one is grassier than the other), have been traveled the same number of times (by other people). The narrator has already said that the second road was "just as fair" as the first (6). Moreover, he says that "[...] the passing there / Had worn them really about the same" (9-10).  In other words, when he says, at the end of the poem, that he will later tell people that "[He] took the [road] less traveled [...]," he will be lying (19). There is no road less traveled; they are each traveled the same.