Accepting that the reality of a story is as described. Magic is real? Sure. FTL? Sure. The colour red makes you fart? Yeah why not? For the sake of the story, I Believe.

BUT the term often gets hijacked by rabid fans trying to justify plot holes, self-contradictions, and nonsensical gobilygook. Suspension of disbelief doesn’t mean I should also suspend all semblance of reason, unless of course there’s a clearly established in-universe justification for doing so.