An ideal blackbody is simply a closed cavity like object which absorbs all incident radiation and re-emits radiation (energy) that is characteristic of the system irrespective of the type of the incident radiation when in thermal equilibrium. The radiated energy is called blackbody radiation.
What is Black Body Radiation?
To stay in thermal equilibrium, a black body must emit radiation at the same rate as it absorbs and so it must also be a good emitter of radiation, emitting electromagnetic waves of as many frequencies as it can absorb i.e. all the frequencies. The radiation emitted by the blackbody is known as blackbody radiation.
