Canon means that it is "Official"
If something is Canon, it means that it is recognized as factual according to the developer of the content.
Non-canon would be something akin to fan-fiction, where you're knowledgeable of a subject and create opinions based on something that may or may not be true, but do not have the credentials to say that it is indeed true about the subject-matter.
In this case, this story is Canon. It is something that is OFFICALLY recognized and ACTUALLY happened in Val's past.
If I were to say Val was also born on the sun, it would be non-canon, because it is not officially recognized that she was born, raised, or has anything to do with a sun and I am just a random nobody on the forums with knowledge of the games, but I am not a developer. Merely a fan with a madeup backstory for a character I appreciate.