What is a black hole? Remains of a massive star that has died. Stars and made out of gas. A reaction occurs and there is a intense gravitational field that pulls things down. As the star dies the reaction stops and at the same time the gravity pulls materials in.
As the core compresses it heats up and eventually creates a surpernova. This is now a black hole. The black hole literally disappears from view. The core of the black hole is called singularity.