The term "Canon" denotes the material accepted as "official". It is often contrasted with, or used as the basis for, works of fanfiction and other fanworks, which are not considered canonical. In the case of South Park, the canon consists of all episodes excluding "City on the Edge of Forever", due to being a dream, and "Pip", due to taking place in Victorian England. The canonicity of the original Spirit of Christmas shorts, and the episode "Terrance and Phillip in Not Without My Anus" is disputable.
Matt and Trey have indicated upon the announcement of the game "South Park: The Fractured But Whole" that they consider the game "South Park: The Stick of Truth", canon to the series and the character Douchebag may be referenced in future episodes.
Canon Pairings are those that have been mentioned or shown in the official TV Program and associated film. For example, the pairing Stendy (Wendy Testaburger and Stan Marsh) is considered a canonical pairing.