To codify is to arrange information in a logical order that others can follow. Legislators may try to codify, or gather and organize, all laws related to a particular issue.
When you look at the word codify you can probably guess that it's related to the word code. Warriors live by a code. Building inspectors check that a building and its systems are up to code. Hockey players use "the code" to determine when — and why — to fight on the ice. All of these codes are clear to the people who use them because someone in the past made an effort to codify the various rules into an organized system.