Canon is official, endorsed work by the creator / author of a piece of work (such as a video game, a book, a movie, etc.)
Alex Rider Canon
The Alex Rider series, film, video games,and related books and short stories are all considered canon. In addition, anything the author states about the series is canon.
Places to Find Canon
The opposite of "canon" is "fanon," anything which is not endorsed by the author / creator and is usually created by a fan.