A hypergiant is super massive star that can be hundreds or even thousands of time larger than our sun( our sun is one of the most common types of stars in the known universe).
Hypergiants have a(luminosity class 0 or Ia+) is among the very rare kinds of stars that typically show tremendous luminosities and very high rates of mass loss by stellar winds. The term hypergiant is defined as luminosity class 0 (zero) in the MKK system. However, this is rarely seen in the literature or in published spectral classifications, except for specific well-defined groups such as the yellow hypergiants, RSG (red supergiants), or blue B(e) supergiants with emission spectra. More commonly, hypergiants may be classed as Ia-0 or Ia+, but red supergiants rarely receive these extra spectral classifications. Astronomers are mostly interested in these stars because they relate to understanding stellar evolution, especially with star formation, stability, and their expected demise as supernovae.