|The comic opens with not just action, but also literary terminology!|
Though one of the more "minor" literary terms, at least as defined by a number of online literary resource guides, an axiom is nonetheless a valuable one to at least be marginally familiar with. In an apparent acknowledgment on the part of the authors that most folks would not be familiar with what an axiom is, they made the decision to overtly provide the answer to the question in the panel itself, "... a fundamental truth that informs their every decision."
In simple terms, an axiom can also be defined as "an obvious maxim" though that particular definition is slightly unsatisfactory as it requires a little additional clarification. If a maxim is "a saying drawn from life, which shows concisely either what happens or ought to happen in life, for example: 'Every beginning is difficult.'" (from Rhetorica ad Herennium), then an axiom is such a saying that is obvious in its truth. Axioms may also be defined a principles that are not questioned, or more simply put (care of Dictionary.com), "self-evident truth that requires no proof; a universally accepted principle or rule."
But, while a definition for the term "axiom" is easily found online, the word "kodan"? Not so much, in fact all I could come across was an inference of a kodan being a narrative device used as part of the Japanese oral tradition... a suitable source for a (likely) manga influenced work of science fiction, especially given that in a previous panel (unseen in the above image) it is attributed to "the Senkarren high priests of Helios..." a science fictiony name if I've ever read one!