So after some googling I found out that the apostrophe in a set means "complement" but in this example it must mean something else right? Aside from that, I have not seen any explanations of the apostrophe in this use case.
"$x$ and $x'$ are distinct elements of $X$" so I wanna know what this $x'$ is.
I've also encountered this kind of notation in another book in a similar case. (something about "bijective")
I'm really sorry that I'm not able to formulate this question any better. I will learn how to formulate it using the tools in my future questions.
Thank you for your time and patience in explaining.