What is a faculty mentor for a junior professor?
It's rapidly becoming the norm at least at larger research universities (R1s) for all junior faculty to have assigned mentors from the senior faculty. At smaller institutions, they may not be assigned and so junior people have to find a mentor themselves from within the college, go externally, use their thesis adviser as a continuing mentor, or choose not to get mentored altogether.

I'm currently serving as a senior faculty mentor to a junior colleague (I was assigned this person by my Chair) and am an informal mentor to two others. As I see it, my roles are to:


Show an active interest in my colleague's work.  Read their published and unpublished work. Go to their internal talks and try to go to their annual conference talks. Provide feedback.
Serve as a soundingboard for my colleague -- which journal should they publish in, should they change the structure of their article, what should their publishing schedule be. Suggest, but don't direct.
Serve as an advocate for my colleague at senior faculty meetings and to the university at promotion and tenure time
Take the colleague out for lunch from time to time. 


In general, try to be a decent human being to them. Especially at the larger R1s, decent human beings are rare so this takes quite an effort. :-)

Note that I've seen a lot of bad mentoring by senior faculty. I'm not sure if they do this because of spite or because they are Evil People®. In my mind, bad mentoring is worse than no mentoring, so I'm not sure if mandatory mentoring programs such as at my institution are a good idea.