Canon is a term synonymous with rules and regulation as to what is acceptable in a fictional universe.
Due to the special circumstances surrounding the Slender Man mythos' creation, there is no official canon or regulation for what constitutes a Slender Man myth, or what is to be accepted as such. This allows ARG's such as MarbleHornets, TribeTwelve and EverymanHYBRID to create and expand on the mythos, and in many ways they have added to the canon more than the original mythos and Slender Man creepypasta.
In relation to this wiki, canon simply refers to something that appears in a commonly accepted Slender Man story, video, etc. If something is non-canon, it means that the thing in question is not related to the mythos. For instance, the Rake, while initially non-canon, makes appearances in EMH videos, and is therefore canon.
Encyclopedia Slenderia contains large amounts of information on Slender Man Canon.