What is code of federal regulation?

The Code of Federal Regulations (CFR) is the set of rules that govern various government agencies and the industries that they regulate.

For an example from my particular field: Congress passed the Food, Drug, and Cosmetic Act (and several related ammendments). This is a law, because it was passed by Congress. That law has several broad requirements. In order to fulfill those requirements, FDA has section 21 of the CFR to spell out how to do so. Regulations are not laws, because they aren't passed by Congress, but "have the force of law", which means that you can still go to jail and pay fines if you break them.

Here's an example: Section 510(k) (also known as 21 U.S.C. 360... don't ask) of the act says: Each person who is required to register under this section and who proposes to begin the introduction or delivery for introduction into interstate commerce for commercial distribution of a device intended for human use shall, at least ninety days before making such introduction or delivery, report to the Secretary or person who is accredited under section 523(a) (in such form and manner as the Secretary shall by regulation prescribe)
(1) the class in which the device is classified under section 513 or if such person determines that the device is not classified under such section, a statement of that determination and the basis for such person's determination that the device is or is not so classified, and (2) action taken by such person to comply with requirements under section 514 or 515 which are applicable to the device.

How you actually do that to FDA's satisfaction is defined in 21 CFR 807 Part E, which is 8 pages long, in Times Roman point 10.

Hope that helps!