What is blackbody radiation?

Hint: All normal matter emits electromagnetic radiation when it has a temperature above absolute zero. It is known as thermal radiation. Conversely, all normal matter absorbs some amount of radiation too. An object which absorbs all amount of radiation of any wavelength into it is known as a black body.

Complete answer:
The term black body was introduced by Gustav Kirchhoff in 1860. Black-body radiation is also called thermal radiation, cavity radiation, complete radiation or temperature radiation.
Black-body radiation is the thermal electromagnetic radiation which operates within or surrounding a body which is in thermal equilibrium with its environment. It is emitted by a black body. A black body is an opaque and non-reflecting body. It has a specific spectrum of wavelengths which is inversely related to intensity that depend only on the temperature of the body
The thermal radiation that is spontaneously emitted by many ordinary objects can also be termed as black-body radiation. A perfectly insulated closed body that is in thermal equilibrium internally contains black-body radiation and will emit it through a hole made on its surface or wall, provided the hole is small enough to have a negligible effect upon the thermal equilibrium.
A black body at room temperature in a dark room appears black due to the reason that most of the energy it radiates is in the infrared spectrum and cannot be seen by the human eye. Since the light waves below the visible frequency are not visible to the human eye, a black body at the lowest, faintly visible temperature appears grey, even though its objective physical spectrum peak is in the infrared range. The human eye cannot perceive colour at low light levels. When the object becomes a little hotter, it appears dull red. As its temperature increases further it becomes bright red, orange, yellow, white, and ultimately blue-white.

Note: Although planets and stars are neither in thermal equilibrium with their surroundings nor perfect black bodies, black-body radiation is just used as a first approximation for the energy they emit. Black holes are near-perfect black bodies, because of the fact that they absorb all the radiation that falls on them. It has been proposed that they emit black-body radiation also known as Hawking radiation, with a temperature that depends on the mass of the black hole.