A fixer is the intermediate between the shadows and other parts of the society. Fixers have numerous contacts, and some also have secondary specialties, such as smuggling, fencing, and so on.
Most Mr. Johnsons, regardless of whom they represent, will contact a fixer to set up a meet with shadowrunners.
Fixers often know many powerful secrets, like the real identities of Mr. Johnsons, the runners, and such. Fixers, however, will not easily part with this information, as secrecy (plausible deniability) is part of the services they provide.