What is National Firearms Act?

Legal Definition
The National Firearms Act (NFA), 73rd Congress, Sess. 2, ch. 757, 48 Stat. 1236, enacted on June 26, 1934, currently codified as amended as I.R.C. ch. 53, is an Act of Congress in the United States that, in general, imposes a statutory excise tax on the manufacture and transfer of certain firearms and mandates the registration of those firearms. The Act was passed shortly after the repeal of Prohibition. The NFA is also referred to as Title II of the Federal firearms laws. The Gun Control Act of 1968 ("GCA") is Title I.

All transfers of ownership of registered NFA firearms must be done through the federal NFA registry. The NFA also requires that permanent transport of NFA firearms across state lines by the owner must be reported to the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF). Temporary transport of some items, most notably silencers, do not need to be reported.
-- Wikipedia