This is a reflection because the pre-image ABC is reflected over a line of reflection, in this case, the x-axis, forming the image A'B'C'. All the points of the pre-image and the image are equidistant from the line of reflection which is a key element of forming a reflection. Also, if you connect A to A', B to B', etc. forming a line, this line's intersection with the line of reflection has to form 90 degree angle.

Information: Reflections