Maybe an example would help?
In Hoard of the Dragon Queen, the first 5E hardcover adventure (and not a particularly good one), the players approach the town of Greenest at the beginning of the scenario. They spot a young green dragon flying overhead. They then go to Greenest to help the defense of the town against the attack of the Cult of the Dragon.
But they're 1st level characters. You've just teased a dragon. If they are rules-lawyer types they know the dragon is far, far more potent than 1st level characters; the lowest draconic Challenge Rating is 4 or 5, if I recall correctly. If they are roleplayers invested in their characters, they know dragons are the most powerful monsters in the Forgotten Realms; they hear stories of heroes contending with dragons... and losing! But the module has no real options for if the PCs decide not to go to Greenest. The advice given is along the lines of forcing them to go to Greenest. Because if they don't the rest of the module doesn't happen.
And that's just... that's just really terrible adventure writing. "It doesn't matter what you yourself think your character would do, the module author will tell you what your character is going to do." What? Why bother going through the rest of the scenario to the final fight? (Where it is heavily stacked against the PCs and designed so that one character will die heroically. What? What if nobody is that flavor of heroic? Not considered in the module. Someone has to die. If the PCs can't make up their minds, the BBEG will pick someone based on these criteria...)
That's basically anti-Player Agency.