Fanon is a term used to refer to "fan canon" (of which the term is a |portmanteau). It applies to certain "facts" that may have been accepted as a truth by a large number of fans, and thus either replaces an established canonical fact in the minds of those fans, or fills a plot-hole.
Fanon can also take the form of personal beliefs held by individuals. In this case, an individual may create an expansive backstory, possibly a fan fiction, which they accept as "true." These have less standing than even the fanon mentioned above, and thus are not included on Tolkien Gateway.