What is the significance mitral insufficiency regurgitation?

Leaky valve. Ekiher term, insufficiency or regurgitation means that the valve is leaking and blood flows backwards. The significance is that it makes the heart work harder and can result in symptoms such as shortness of breath.
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.