The mitral valve separates the left ventricle from the left atrium. It acts like all valves in the heart-- as a one way valve. This allows the blood to flow forward when the heart contracts and prevents leaking back into the left atrium as the heart relaxes between each contraction. The valve is composed of two leaflets which look, with the eye of faith, like the mitre the Pope wears-- hence the mitral valve.
The valve can become leaky (mitral regurgitation or MR) and this can cause the blood to leak into the left atrium.