The defining data of glide reflection is a line m, called the mirror, and a vector V, called the glide vector. The vector V must be parallel to the line m. If we use an ordered pair of points A,B to determine the vector V, then the line AB must be parallel to the line m.
If the point X is on the mirror line m, then X' is just the translation of X by the vector V.
If the point X is not on the mirror line m, then X' is obtained from X by first translating by V and then reflecting across m. In other words, there is a point Y so that AXYB is a parallelogram and m is the perpendicular bisector of the segment X'Y.
(It follows that if Z is the reflection of X across m, then XYX'Z is a rectangle. Thus X' can also be obtained by first reflecting X across m and then translating by V.)