Question

what is meant by X'X in the following image ​

Answer 1

A number line / axis / dimensions and many more coordinate systems are represented by using X'X, Y'Y and Z'Z if there are 1,2, or 3 dimensions in use.

Here, the numbers below 0 or negative values are the ones which exists closer to X' end of the line while X possesses +ve value. and O or origin is the exact center.

By X'X he means to say every value consisting but not limited to the values between X' and X.