Ideal object that is a perfect absorber of light and also a perfect emitter of light. A perfect black body will absorb all radiation that falls on it, and will emit radiation that has a continuous spectrum determined only by the temperature of the black body:

Where, T is the temperature (K), λ = wavelength (cm), c = speed of light, k = Boltzmann’s constant (1.37 x 10-18 erg/K), and h = Planck’s constant (6.626 x 10-27 erg sec).
Light is emitted by solid objects because they are composed of atoms and molecules which can emit and absorb light. They emit light because they are vibrating due to their heat content (thermal energy). The distribution of energy radiated against wavelength follows the Planck curve and, for a given temperature, there is a particular wavelength at which the maximum emission takes place specified by Wien’s Law. Stars and hot solid bodies are not perfect black bodies, but their radiation can be described in terms of black-body properties.

Image Source: Figure 3.8, Introduction to Modern Astrophysics, 2nd ed., Carroll & Ostlie, 2006.