A constant factor is a variable that cannot be changed. Some are purposeful and selected by the scientist to control the experiment while others are universal and beyond the scientist’s control. If the scientist does not note the constants, then these can lead to perceptual errors in the experiment.
So too with your story world. The physical rules of your story world, both physical and social should remain constant. Your characters will be the only variables. Establishing the story world’s constants at the beginning of the story allows the reader to focus on what really matters: the story.