Reflection is the phenomenon that occurs when waves reach the interface between media of different refractive index. To ilustrate it, we will use the most commonly observed case, light bouncing off a mirror.
In the diagram above, a light ray PO hits a mirror (vertical line) at point O, and bounces off it as ray OQ. By projecting an imaginary line through point O perpendicular to the mirror, known as the normal, we can measure the angle of incidence, θi and the angle of reflection, θr. The law of reflection states simply that θi = θr, i.e. the angle of incidence equals the angle of reflection.
In fact, reflection of light may occur whenever light travels from a medium of a given refractive index into a medium with a different refractive index. In the most general case, a certain fraction of the light is reflected from the interface, and the remainder is refracted. Solving Maxwell's equations for a light ray striking a boundary allows the derivation of the Fresnel equations, which can be used to predict how much of the light is reflected, and how much is refracted in a given situation.