What is the job of the mitral valve?

Prevents backflow. It prevents blood from going backwards from the left ventricle into the left atrium with each heart beat.
Mitral valve. The mitral valve has to open properly to let blood flow into the left ventricle from the atrium. If it doesn't, we call this stenosis. It has to close to keep the blood flowing toward the body, if it doesn't we call this regurgitation or insufficiency. Either or both problems can exist with the valve if its abnormal. Insufficiency is more common.