It means very large, very abundant, very concentrated. In High Mass and Very Dense Astronomy. It can also mean in very invasive microbiology, which infects very quickly or very aggressively (referred to a virus).
It has a very large and concentrated mass. It is used in astronomy for the description of a black hole.
Superlative of massive. Which is exaggerated in large quantities. In Astronomy supermassive black holes are those that have a mass of millions of solar masses. There is a theory quite supported by astronomers that there is one at the center of our Milky Way. Today, October 2020, astronomers have found six galaxies around a supermassive black hole when the Universe was less than 1 billion years old.