First, we consider reflection, as shown in the diagram below for a light wave striking a surface. We identify the incoming ray as the incident ray and the outgoing ray as the reflected ray. Concomitantly, the angle that the incoming ray makes with a line (dashed in the diagram) normal to the surface is called the angle of incidence. The angle for the reflected ray is called the angle of reflect.
The law of reflection states that the angle of incidence is equal to the angle of reflection, or, stated mathematically.
And also the incident ray, the reflected ray, and the normal to the surface of the mirror all lie in the same plane.