When the Mitral Valve can not open effectively it is said to be stenosed. In other words, the valve closes off as a result of a disease, or because a person was born with a Mitral Valve which does not completely open. The difference between these two examples are what separates people with acquired valve disease (ie having an infection that affects the valve) and congenital valve disease (ie born with an abnormal valve).
In either case, the event causes the valve to swell resulting scaring as part of the healing process. The scarring is what causes the valve to stiffen and ultimately close off.
Mitral Valve Stenosis produces turbulence as blood passes through the valve. Turbulence is what results in a murmur which typically is heard during the phase of the cardiac cycle called diastole (REMEMBER! WE DEFINED IT EARLIER IN THE POST ABOUT MITRAL VALVE FUNCTION!)
Watch in this short (30 second) video how the Mitral Valve fails to completely open when compared to the valve at its side. Also pay close attention to the sound of the murmur during diastole.