In traveller terms this means:
Anything Marc says is canon.
Anything published is canon.
But some say that GT is alternate history…
Some really strange stuff, like Paranoia’s “Beyond Sector” has been decanonized, and is not considered canon anymore.
Other really bad stuff, like some of the stuff that imperiumgames produced, is just considered rubbish…

The important thing if You publish anything for traveller, and want the traveller community to like it, is to try to follow canon. If not, there will always be some canonheads that will tell You that Your stuff stinks…

There are also some troubles with canon: (Maybe interesting new subjects for this board.)
One that is often discussed is that the CT book-2/5 has a 100 ton minimum limit for jump capable ships, but in CT adventure Leviathan, small jump torpedoes are used.

Other problems are: Piracy, Kinunir, Annic Nova, Reno, Different types of M-drives, Fusion plus, The Empress Wave, Solomani Migrations, SDBs, The Gazelle Class, Drop tanks, etc, etc…