The Schwarzschild metric was calculated by Karl Schwarzschild[1] as a solution to Einstein's field equations in 1916. Also known as Schwarzschild solution, it is an equation from general relativity in the field of astrophysics. A metric refers to an equation which describes spacetime; in particular, a Schwarzschild metric describes the gravitational field around a Schwarzschild black hole - a non-rotating, spherical black hole with no magnetic field, and where the cosmological constant is zero.
It is essentially an equation that describes how a particle moves through the space near a black hole.