A blackbody is an idealised* object that absorbs all forms of radiation from the electromagnetic spectrum, and more importantly, does not reflect any of it. Since (visible) light is part of the electromagnetic spectrum, that means it absorbs all light, and reflects none, making it completely black (black/darkness is the absence of light being reflected), as such it is called a blackbody.
The idea of a black body can be understood by using a hypothetical box, with a tiny pinhole inside:
*Blackbodies are hypothetical objects, but there are a lot of objects that have an emission spectra similar to that of a blackbody, such as celestial objects. #tosee [Why hypothetical?]
A blackbody aims to be in thermal equilibrium with its environment. If the blackbody is cooler than its environment, it absorbs radiation, warming it up. However, if it is hotter than its environment, it emits radiation, cooling it down. The steady state is thermal equilibrium, where the rate of absorption and emission is the same, and the temperature of the body stays constant.
Thermal Equilibrium
Since a blackbody is defined to not reflect any electromagnetic radiation, it can be considered more ‘efficient’ than a normal object:
If we look at a blackbody in thermal equilibrium with its environment, we get another useful property: It also becomes a perfect emitter.
Why? As defined before, a blackbody is an object that absorbs all incoming radiation. The definition of an object in thermal equilibrium is that its absorption and emission is the same. As such, combining these two properties means a blackbody in thermal equilibrium is a perfect absorber (of radiation from the environment) and a perfect emitter (emits the same radiation back out).