Waves - including sound and light - can be reflected at the boundary between two different materials. The reflection of sound causes echoes.
The law of reflection states that:
angle of incidence = angle of reflection
For example, if a light ray hits a surface at 32°, it will be reflected at 32°.
The angle of incidence and angle of reflection are measured between the light ray and the normal - an imaginary line at 90° to the surface. The diagrams show a light ray being reflected at a plane mirror.