It means that applying techniques like small world, obdurium walls, false choices, or Schrodinger's maps can be justified if the GM is doing so because it's impossible to prep a fully open world with infinite choices and also have real granularity about it.
For example, in my own campaign right now I'm running a Star Wars bounty hunter game set in the rise of the empire. The convention in the game is that the guild master Yagmur offers the players a job to acquire some high value bounty. In reality, this is a false choice. I don't really have anything prepared if they say "no", and my players understand this and take the hook. I don't like this, but it's a necessary convention to run the game. In the ideal world I'd allow players to log into the Imperial database and browse through the 100's of thousands of bounties, filtering them by sector and value and type of bounty and let them select the job they are most interested in, and no matter what they selected I'd be able to run an interesting game for them. But the constraints of my ability to provide this much content just prevents that true open world. So I have to run some sort of narrow-broad-narrow structure where they are given a mission, they go learn about that mission in a manageable environment with a limited number of points of interest, and they explore that environment in the order of their choosing, collecting clues and likely getting into trouble, and then as a result of their investigation they figure out one of the locations they could find the acquisition at and arrange some sort of heist to assassinate, kidnap, rescue or whatever else they need to do to acquire the target. That's a small world. It feels pretty open but it has some walls around it. I don't actually detail whole worlds with billions of inhabitants and if you get off the map details will be improvised and sparser than what you would get if you stay in the expected portion of the sandbox.
Generally speaking as a player I would also play along with this sort of thing for the good of the game rather than trying to actively derail it, especially if I am having fun. I understand the limits of preparation and imagination put limits on what can be supported and I'm not going to fault the GM for that.
However, if a GM has rails that are too clunky, overtly involve the GM telling me what to do in director stance, require certain choices by me to stay on them, is railroading because they have expectations about what the players will do or how certain scenes will play out (like the villain will escape in Scene B), or if they actively try to prevent me from going from A->C because the plot they created absolutely depends on me going through the exact same hoops (often stupid hoops), then I'm much less likely to be forgiving.