"Briefings" were a technique used by the Great Houses during the War in Heaven to recruit and indoctrinate members of the lesser species by providing them with basic background information about the main players in the War as well as a strong sense that the individual's actions are necessary for their life to continue.
The Great Houses notably used briefings to insure that soldiers would go into psychological breakdown and commit suicide if they ever had sympathetic thoughts about another power. Regen-inf troops were also commonly given physical self-destruct protocols.
The Book of the War said that "briefed" individuals were humanity's main source of information about the War and that briefings were "absolutely and unequivocally necessary if anything resembling normal life is to continue", echoing its description of the feelings induced in soldiers by briefings. However, the Shift cast doubt on that statement. (PROSE: The Book of the War) The Book of the War was itself a briefing. (PROSE: The Book of the Enemy)