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Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body. It has a specific, continuous, spectrum of wavelengths, inversely related to intensity, that depend only on the body's temperature, which is assumed, for the sake of calculations and theory, to be uniform and constant.
A perfectly insulated enclosure, that is in thermal equilibrium internally, contains black-body radiation, and will emit it through a hole made in its wall, provided the hole is small enough to have a negligible effect upon the equilibrium.
The thermal radiation spontaneously emitted by many ordinary objects can be approximated as black-body radiation.
Of particular importance, although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is a good first approximation for the energy they emit. The sun's radiation, after being filtered by the earth's atmosphere, thus characterises "daylight", which humans have evolved to use for vision.