The idea comes from Inquisitor, an older rule set made for the 40k universe, that looked at the world through a more individual level. Small gangs of people where the story arises from the people involved rather then planet shaking armies of nameless billions fighting over some mystical tat.

Essentially a tabletop wargame with a looser rule set to accommodate a greater focus on narrative then being competitive.

It is by no means restricted to 40K alone or it's typically dark themes/imagery. Practically anything will do. It all depends on what the participants want out of it.

This link gives a more detailed description of what it's all about.

Somethin' or other ain't what it used to be, ain't what it used to be, or maybe it still is.