Black Holes are classical gravity objects as predicted by solutions of the Einstein-Hilbert Field equations. They carry the form:
which goes wrong when . This point is called the "horizon" of the black hole. At this point (where r is measured from the black hole) the metric becomes all space-only and the time coordinate is killed. This is the Schwarzschild Black Hole, invented by Arnold Schwarzschild after 1915. Other Black Holes were later invected that hide charge and can spin, and can even be "extended" by black branes.
Black holes have never been seen. It is claimed by some that black holes exist in the centre of stars and can "evapurate" after a long time (see Weak Gravity Conjecture). If black holes exist, then they predict many warped effects of physics such as time dilation, which stops completely at the horizon boundary; thermal entropic radiation; virtual particle creation at the horizon; etc.