What are supermassive black holes?
It is literally supermassive.
Black holes are formed when a star dies out. It shrinks to it Schwarzschild radius which is really very small.
For example, if you want to make earth a black hole, (Don't ever try this!) you have to compress it to the size of ping pong ball. That's Earth's Schwarzschild radius.
Supermassive black holes are huge in size. We know even a small black has a very intense gravity. Supermassive black hole have unexplainable-ly intense gravity covering a very large radius of attraction.
They are mainly located in the centre of a galxy. In our Milky Way's case, it is named as Sagittarius A*.