Player Agency is a weird term, but it really boils down to the player being able to make meaningful decisions that impact the game and/or world. These can be in big or small ways, but players like to know their decisions actually matter.
A whole lot of people take that or a similar position. I have come to feel it is incomplete.
Specifically, this focuses player agency only within instances of play, and only on the players direction of their character. But, really, player agency is larger than that - player agency is also exercised when choosing what playstyle to engage in, what game to play, and so on. Player agency starts before play, back in Session Zero.
So, when a player chooses what GM and player group to work with, or not work with, they are exercising their agency. If players choose to take on what they know is going to be a fairly linear adventure, in which they aren't going to have many choices that will impact the result, they are still exercising their agency! How they build characters and backstory is an exercise in agency.
When players discuss among themselves what they do and don't want in a game, and negotiating what the campaign will be like - yep, exercise of agency.
Choosing to allow someone else to take a spotlight moment is an exercise of agency.
And so on. There's lot of places where players make choices that shape the game that aren't about what actioin is taken in the fictional world.
Edit to add: Even in the most railroady adventures, there's agency to be found in HOW characters address challenges. So, maybe the adventure sets up an encounter with kobolds in which a diplomatic solution is really the only option. But some parties will draw up treaties, and others will wind up with their dragonborn paladin as the new Patron Saint of kobolds.
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