A quantum is the smallest bit of electromagnetic radiation that can be emitted. It is also called a photon of light or small packet of electromagnetic radiation.
Basically, light and matter at the subatomic level display characteristics of both waves and particles. De Broglie found that the velocity of these particle when confined to a small region of space must equal n(h/2Lm) where n is a positive integer and h and L are related to wavelength. Therefore, the particles only move at specific speeds determined by n. These possible values of a particle's energy are called energy levels. The integer, n, is called the quantum number.