Plain English Meaning
The CFR is the master rulebook of the U.S. federal government. The aviation rules pilots must follow live inside it, in Title 14.
Full Definition
The Code of Federal Regulations is the official, organized collection of all permanent rules issued by the executive departments and agencies of the United States federal government. It is divided into 50 numbered Titles by subject area; aviation rules are found in Title 14, which contains the Federal Aviation Regulations (FARs) that govern pilots, aircraft, airspace, operations, and certification.
Context Anchor
You will see CFR references in FAA handbooks, regulations, checklists, training materials, and discussions about what a pilot is legally allowed or required to do.