People say all kinds of stuff during play. The question is who gets to say what so that it sticks. Meaning, we're going to proceed from this point having accepted what was said into what's going on.
When you cut the goblin's head off, do you look at the dice and whatever, and say so? Or does it not quite count until the GM says so?
Four distinct types
Content authority - over what we're calling back-story, e.g. whether Sam is a KGB mole, or which NPC is boinking whom. This includes preparing such information at any point as well as revealing it in play.
Plot authority - over crux-points in the knowledge base at the table - now is the time for a revelation! - typically, revealing content, although notice it can apply to player-characters' material as well as GM material - and look out, because within this authority lies the remarkable pitfall of wanting (for instances) revelations and reactions to apply precisely to players as they do to characters.
Situational authority - over who's there, what's going on - scene framing would be the most relevant and obvious technique-example, or phrases like "That's when I show up!" from a player.
Narrational authority - saying how it happens, what happens - I'm suggesting here that this is best understood as a feature of resolution (including the entirety of IIEE), and not to mistake it for describing what the castle looks like, for instance; I also suggest it's far more shared in application than most role-players realize.
See also Credibility.
Distributed vs. centralized
Historically, role-playing culture assigns one specific person most or all of all four types of Authority. Such an arrangement is viable, but the assumption that the four types must be held by one person is not. Holding that assumption while not actually applying it in play can cause trouble at the Social Contract level.
Related issue: The Good GM.
Fixed vs. changing