There are 4 heart valves, 2 on the right side and 2 on the left. There is one between the two chambers on the right (the tricuspid valve), and one between the two chambers on the left (the mitral valve). There is one at the exit of the right heart (the pulmonary valve) and one at the exit of the left heart (the aortic valve).
Three of the valves (the tricuspid, the pulmonary and the aortic) have 3 leaflets, one (the mitral) has two. As with all valves, the role of the heart valves is to ensure that the blood goes in one direction and one direction only.
The valves may be INCOMPETENT (meaning that they leak, ie that they don’t seal properly) or STENOSED (which is medicalese for narrowed). Reasons for heart valve problems are most often due to infection (bugs in the blood), but can be congenital. The problem is that narrowing of the valve or leaking of the valve results in blood going the wrong way. Effectively, then, the bit of the heart behind the valve has to work harder. Eventually, that hard work causes the heart muscle to give up (or fail).
Valve replacements can be entirely artificial (ie metal, usually titanium because it’s hard and doesn’t tarnish) or biological (from another human being who, of course, is dead) or even from another animal (usually a pig because pigs and humans have a lot in common).