This heavy core of the supernova star continues to contract (shrink) and becomes a neutron star. The fate of this neutron star depends on its mass. If the neutron star is very heavy, then due to enormous gravitational attraction, it would continue to contract indefinitely (or continue to shrink in size indefinitely). And the vast amount of matter present in a neutron star would be ultimately packed into a mere point object. Such an infinitely dense object is called a black hole. Thus, black holes are formed by the indefinite contraction of heavy neutron stars under the action of their own gravity.
The neutron stars shrink so much and become so dense that the resulting black holes do not allow anything to escape, not even light, from their surface. This is because the black holes have tremendous gravitational force. Since even light cannot escape from black holes, therefore, black holes are invisible, they cannot be seen. The presence of a black hole can be felt only by the effect of its gravitational field on its neighboring objects in the sky.
For example, if we see a star moving in a circle with no other visible stars in the center, then we can conclude that there is a black hole at the center. And it is the gravitational pull exerted by this black hole which is making the star go in a circle around it. Thus, if the sun were to become a black hole,