When a star dies, it can transform into a black hole. Whether or not this will happen depends upon the mass of the star. If the mass is at least five times that of our Sun's mass, the star becomes a black hole.
When a star is shining, its core generates energy using nuclear fusion. This pressure prevents the star from collapsing against its own gravitational field.
When the star dies, it collapses under its own weight. A large amount of mass gets confined to a very small region of space, resulting in a very high gravitational field.
Nothing can escape the gravity of a black hole. It consumes the planets and stars that come into contact with it. Even light cannot escape the powerful field of a black hole.
Many galaxies have supermassive blackholes at their centers. Our own Milky-way galaxy has a black hole that is about as heavy as 4 million Suns. A black hole can be as heavy as 50 billion Suns.
Interesting things happen when two black holes come near each other.
They circle around each other for a few million years like two lions sizing each other up. As they come closer, their speed increases until they merge and form one single black hole.
Collision of two black holes is one of the most violent events in the universe.
The energy they sent out is in the form of waves. These are the gravitational waves. In 2017, detection of gravitational waves won the Nobel Prize in Physics.