A black hole is a very heavy mass of very high density in space. The attractive force of gravity on the black holes is so high that no object and even the light rays cannot escape from it. Since a black hole does not emit or reflect any light, it remains unseen and is a black or dark region in the space.
Suppose a very heavy star (many times heavier than our Sun) has cooled below a temperature that is required to keep its atmosphere stable. The matter on the star is so heavy and dense that gravity pulls in every matter inside. Suppose the compression of matter on the star continues, then it shrinks many times its original size and becomes a black hole.
In our galaxy of milky way, there are many black stars present. Their presence has been detected and observed by scientists. We cannot detect a black hole directly. We can see some stars are revolving around unseen black hole. We find that cosmic radiation comes in from every where except some regions, which are black holes.