Music in the style of heavy metal or one of its sub-genres that is written, performed, and recorded for no other reason than that it rules. Thus, heavy-metal-style music written specifically for commercial success cannot be considered "true," and is instead
false metal. This does not necessarily mean that the false metal is not enjoyable to listen to, but it does mean that it is
not metal.
It is possible for bands that were once true to become false, most famously Metallica. The reverse is also true, although it is much rarer.
A good rule of thumb in distinguishing
true metal from false metal is to check Metal Archives. If a band/artist is listed, they either are or were true metal.
The term is also occasionally used to describe old-school, straight forward heavy metal, as opposed to one of the sub-genres.