Typically ‘x marks the spot’ refers to where the treasure is buried on a map, however, we’re using it more to indicate the ultimate goal (or maybe even the side goals) of your story. What is your character/s X on the map of the plot?
The thing that we’ve found with writing stories is that they aren’t as concrete as burying treasure. So, your X may end up a hundred miles away from where you intended it to be, but as long as you’ve followed the signposts, you’ve placed yourself, then the result should still be true to your original framework
Of course, we’re not going to politely inform you of our personal X’s, mostly because due to the number of works we have, you’d be reading for a week and well, no one’s going to do that.