In the universe you can find many blackbodies. The sun and the stars are termed as black bodies. The radiation the black body emits is termed as the blackbody radiation. This radiation can be in any wavelength of the electromagnetic radiation from the infra red to visible light to ultra violet rays. The body that has been incident with some rays of electromagnetic radiation will absorb all of it.
Then it converts it into heat also called internal energy. This is then reflected to its surroundings. This is the blackbody radiation. This radiation is solely dependent on the body's temperature. For example, for a particular temperature a specific wavelength is emitted by the body. The blackbody radiation was not much understood way back in the eighteenth century.
Many physicists have worked on this theory like Max Planck, Stefan, Boltzmann, etc. to name only a few. They have deduced this property of matter in detail and put forward certain theories and framed certain laws. We have the Max Planck's theory and the Stefan-Boltzmann law and so on that were figured out from the various research work independently by these scientists.
Also some formulae were worked out of this research work which became the basis of certain calculations in the future. The theory of blackbody radiation was mostly helpful to find the temperature of the surface of various stars which was otherwise difficult to construe at that stage when not much about the space or the atmosphere of the earth was known. It was also figured out after a lot of research that every star or body has an effective temperature. This theory was made use of to find the stellar temperature. The black body temperature determines the energy that is radiated from it.
In other words the amount of energy or heat that is emanating from a blackbody or a star can help us know the temperature at which it was emitted. Thus the surface temperature of several heavenly bodies mainly stars including our sun was calculated.
There are other uses and applications of this theory. At the time when this theory of radiation of the black body was discovered the atomic theory of particles was not fully known and it was at that time deduced based on the facts known at that time. Yet most of the facts discovered then hold good even today. This theory helped in understanding the theory of reflection of light by the surface of bodies. Later when the atomic theory was unraveled most of the lapses in this theory were understood.
Thus we can say that blackbody radiation was useful to know the yet unknown facts about matter in particular and physics in general.