Black holes are one of the most mysterious and powerful forces in the universe. They are formed when giant stars explode at the end of their lifestyle. This explosion is called a supernova. if the star has enough mass, it will collapse on itself, down to a very small size. Due to its small size and enormous mass, its gravity will be so strong that nothing can escape from it- even light. Instead, all light is absorbed, and a black hole is formed.
Black holes can grow incredibly huge as they continue to absorb light and mass around them. They can even absorb other stars. We can't actually see black holes because they don't reflect light, but scientists know they exist by observing light and objects around black holes. A simple way to put it is that a black hole is a point in space where something exists, but because of its extreme gravity, light cannot escape from it, and so, it is not visible under any conditions.