It is a personality tool to define what a character believes in. There are 73 codes outlined here, 146 is you include anti-codes. These are the individual, personal rules that defines what a character believes in; what they live for...what they would die for.
Codes are defined as simply as possible so you can start putting this powerful tool to use right now!
Take the extra step in character desihn that separates a simple piece of paper from a vivid character that lives and breathes in your imagination.