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 absorb 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.
In the Series 4 story "Turn Left", Rose used the phrase to identify herself to the Doctor (via Donna) & to warn him of the danger to the existence of the universe posed by Davros & the Daleks (as subsequently revealed in "The Stolen Earth"/"Journey's End").
For further information, see Bad Wolf & the various articles listed there.