Energy striking any object from it's surroundings, can be absorbed by the object, reflected by the object, or transmitted through the object (if not opaque). If the object is at a constant temperature, then the rate at which it emits energy must equal the rate at which it absorbs energy, otherwise the object would cool (emittance greater than absorption), or warm (emittance less than absorption). Therefore, for bodies at constant temperature, the emittance (absorption), the reflection and the transmittance of energy equals unity.
The concept of the blackbody is this: a blackbody is a theoretical object that neither reflects or transmits, but absorbs all incident radiation, independent of direction and wavelength. In addition to absorbing all incident radiation, to maintain a constant temperature, the blackbody must also re-emit as much radiation as it absorbs - a perfect radiating body.
When a blackbody is heated it emits radiation of many wavelengths. The energy density as a function of wavelength varies at different temperatures of the blackbody. If one were to plot the energy density vs. wavelength for various temperatures you would have the blackbody radiation spectrum.
Blackbody Radiation Explained - Quantum Mechanics
Blackbody Radiation Simulation
Simulator courtesy of University of Nebraska-Lincoln