What's a simple way to say what mitral stenosis really is?

Narrowing: Mitral stenosis is narrowing in the valve orifice. This abnormal valve doesn't open properly, blocking blood flow coming into your left ventricle, the main pumping chamber of your heart.

Narrow gate: Narrowing of the gate between the left upper and left lower chambers. This prevents blood from freely entering the lower chamber thus reducing amount of blood available to be pumped causing fatigue.Also there is build up of back pressure hence water logging of lungs, liver, abdomen and limbs causing shortness of breath, nausea, poor appetite and leg swelling. Symptoms depend on degree of narrowing.