What is a leak in the mitral valve?

Backward flow: The leaflets of the valve are not closing perfectly and some blood goes backwards from the left ventricle into the left atrium. Minor leaks are common and benign.

Usually benign: Most of this is something called mitral prolapse which is often an almost totally benign process. Sometimes, the support structures for the valve (called chordae) rupture and the valve flails like a parachute with cut cords. That leak has to be fixed surgically, as does one caused by bacteria eating holes in the valve.