Background Count is, basically, Astral Smog. It's the twisting of astral space towards, or against, the flow of mana.
Various things affect the Background Count, ranging from strong emotions, history, to just plain the unknown.
Example: A bar like
Freeloadin' Freddie's Fighthouse would have a positive Background Count as people are having fun, living life, enjoying themselves. Sure it's violent, but it's a friendly violent. That's part of life.
A bar like
Hookers and Boomsticks would have a negative Background Count because the clientele are there mostly Humanis Policlubbers, who like to share in hate and prejudice.
So, let's say they both have a Background Count of 2.
An Urban Shaman and a Toxic Shaman are calling up magic for fights in both bars (It's a roving fight). While in Freeloadin' Freddie's, the Urban Shaman is able to tap into the Positive Background Count and gets a +2 to his Magic Stat, while the Toxic gets a -2 to same.
Now, the fight heads on into Boomsticks and Hookers, where the concentrated Hate helps out the Toxic, and he gets +2 to his Magic Stat, and the Urban Shaman is at -2.
PS: Oh, yeah... Street Magic, Page 117.