DForck42's answer explained how it appeared, but not exactly what it means.
The phrase 'Bad Wolf' came from the name of the corporation which the Daleks have taken over and owned the games satellite they were on at the start of the episode Bad Wolf:
And the message was therefore meant to tell her that she should return to the Bad Wolf Corporation satellite to go and save The Doctor.
At the end of The Parting of The Ways, Rose also explains it further:
Rose: I am the Bad Wolf. I create myself. I take the words...
[Rose lifts her hand and takes the words from the Bad Wolf Corporation sign]
Rose: ...I scatter them, in time and space.
[the words float off away from them]
Rose: A message, to lead myself here.