If we consider a horizontal axis centered at the line of reflection, each vertice of the quadrilateral A'B'C'D' has the horizontal coordinate opposite to the corresponding vertice of the quadrilateral ABCD.
Another way of understanding the relation between a point on the original figure and its image on A'B'C'D' is the following:
The vertices of quadrilateral ABCD and the corresponding vertices of quadrilateral A'B'C'D' are equidistant from any point on the line of reflection.
It means that, if we choose any point on the line of reflection, the distance from this point to A equals the distance from this point to A'. And the same is true for the other vertices.
Notice that the same holds for any point on the original figure and its image on A'B'C'D'.