High densed mass of which gravitational force is much much large.. It can absorb light also and can bend the path of a light ray, which is travelling besides it..
If the mass of a star is 5 times the solar mass then during its death it will shrink due to tremendous gravitational force towards its center. It will convert into a mere , very dense and dark matter. This is black hole.
It is a place or whatever i don't know , which has such a large gravity that even light cannot comes back.
A star has very large gravity..but still it doesn't collapses because of the nuclear reactions going inside it...but when it runs out of fuel it starts collapsing....gravity is inversely proportional to square of radius so when stars start collapsing the and the gravity starts increasing...if the initial radius was very large then the final gravity is extremely large...this results in formation of black hole.