Personal canon is one fan's interpretation of canon. This can range from picking and choosing stories and or characters that he or she favors and assembling them as he or she likes, or discarding one or two distasteful properties from an otherwise accepted continuity.
Personal Canon is what is often used in fan fiction, where an author will use an established continuity up until a certain point, from which he or she would start telling their own stories. Motivations for these stories can range from a desire to retcon another story that the author dislikes, or to curiously pursue a 'what if' continuity based on a key event happening in some different way.
On a more simple level, personal canon can refer to an individual fan's overall preference for certain interpretations of the Ghostbusters story over another, such as saying "Stay Puft is cool, as long as the Ghostbusters don’t keep him in their Containment Unit”