When light hits an atom, it causes the electrons to jump from a lower stair, or lower energy level, to a higher stair, or higher energy level. The atom only absorbs light if it is of an energy that corresponds to the height between the lower and higher energy level. Similarly, when atoms are very hot and bouncing into one another, their electrons are excited into higher energy levels. Thus, each type of atom has a unique way of absorbing and emitting light. The colors of light that an atom emits are called its emission spectrum, and the colors it absorbs are called its absorption spectrum.
All objects continuously emit light of some wavelength or another.
This radiation is usually outside of what humans can see. It is called blackbody radiation because it refers to light emitted by an ideal object that is a perfect absorber and emitter of radiation. When objects are hot enough to emit light that is visible to humans, we can easily tell its relative temperature. As the object increases in temperature, it goes from red to orange to yellow to blue. The wavelength of the most common light emitted by the hot object is called the wavelength of maximum intensity.