From the Introduction:
Alignment is a way of describing the moral code of a creature. This supplement is intended to help the Game Master create a system of alignments for his or her own game world. These rules are for use with the Basic Fantasy Role-Playing Game rules.
Alignment, in a role-playing game, refers to a system of defining ethics and morality in the campaign world. The very first role-playing game possessed an alignment system, basically defining “sides” to which the heroes and monsters belonged. Later games have expanded on this concept in several details. How alignments are defined, judged, and applied in the game world has long been a point of contention