What is the significance of a narrowed heart valve?

Stenosis . Blood will have trouble going between heart chambers and great vessels. Depending on the chronicity and the degree of stenosis, bad damage can happen to your lung circulation and your heart chambers. Consequences can be catastrophic. Heart attacks, syncope, heart failure, arrhthmias, sudden death. Common causes are uncontrol htn, congenital bicuspid aortic valve, infections/iv street drugs etc.
Restricted flow. A heart valve that is narrowed has a restriction on the amount of blood that can flow across it as the heart pumps. This restriction, often referred to as a "stenosis" can vary from very mild and insignificant to life threatening. A valve is usually measured by the area of the valve. As that area decreases the flow likewise decreases. When diagnosed early, many stenotic valves can be fixed.