From the Introduction:
Alignment is a way of describing the moral code of a creature. These rules are for use with the Basic Fantasy Role-Playing Game rules.
Basics of Alignment
A creature’s general moral and personal attitudes are represented by its alignment: lawful good, neutral good, chaotic good, lawful neutral, neutral, chaotic neutral, lawful evil, neutral evil, or chaotic evil.
Depending on the campaign, the GM may require that players choose an alignment for their characters. Alignment is a tool for developing your character's identity. It is not a straitjacket for restricting your character. Each alignment represents a broad range of personality types or personal philosophies, so two characters of the same alignment can still be quite different from each other.