The Superman Mythos refers to the collection of concepts, characters, stories, and ideas from various versions of Superman's storyline. Everything you have ever seen, read, or heard about Superman is a part of Superman's Mythos.
Unlike other science fiction franchises, Superman is divided into many different Continuities. For example, Smallville (TV) is not continuous with any other Superman comic book or movie-- that is, events that happen to Clark in Smallville may not have happened to the Superman you see in the movies. As such, the only thing the Superman franchise as a whole has in common is the mythos. So, while Smallville isn't continuous with other sources, it does draw from them: The concept of the show is based on the Superboy (comic series), while the show's version of Krypton is based on the Movies' version, and certain story arcs are based on modern age comics.
Every official Superman source adds to the Superman mythos. The comics, the TV series, the movies, the cartoons, and more. And each new source that comes out draws from the mythos in general. Even the comic books, which are widely credited with being the "main" source; they draw on elements originally from Smallville, and from Superman: The Animated Series, and the Superman movies, etc. to create their version of the world of Superman.
The Superman mythos has evolved continuously since Superman began in 1938.