Technically, it doesn't mean anything. "Bad Wolf" is an ontological paradox wherein the words were scattered across space and time by Rose Tyler after she absorbed the energy of the time vortex as a way to ensure that she would always be led to that point. By spreading the words "Bad Wolf" across history, her younger self would encounter them during her adventures with the Doctor and would eventually follow them to the point where she would look into the Heart of the TARDIS and absorbed the time vortex, and subsequently scatter the words.
Rose explained this in the speech in "The Parting of the Ways" that began "I am the Bad Wolf. I create myself..." -- a rare case of a companion explaining to the Doctor what's going on, instead of the other way round.