The Kerr metric describes an uncharged black hole with what property? The answer is rotation.
The Kerr metric is the exact solution of Einstein’s general-relativity equations for the spacetime around a rotating, uncharged black hole. It was discovered by New Zealand mathematician Roy Kerr in 1963. The solution describes a black hole characterized by mass and angular momentum, with electric charge set to zero.
Rotation changes the surrounding spacetime. A spinning black hole drags nearby spacetime around with it, an effect called frame dragging. The region outside the event horizon where this influence becomes unavoidable is called the ergosphere. In principle, energy can be extracted from the ergosphere through the Penrose process.
The nonrotating, uncharged idealized black hole is described instead by the Schwarzschild metric. Adding electric charge produces the Reissner–Nordström solution if the black hole does not rotate, or the Kerr–Newman metric if it rotates. Real astrophysical black holes are expected to spin, so the Kerr solution is especially important in modeling accretion disks, jets, and matter orbiting close to the horizon.