The structure indicated is the tricuspid valve of the heart.

There are four sets of valves in the heart. Between the atria and the ventricles are the atrioventricular valves; on the right is the tricuspid valve, on the left is the mitral (bicuspid) valve.

Between the ventricles and the aorta/pulmonary trunk are the semilunar valves. The pulmonary valve is located between the right ventricle and the pulmonary trunk, and the aortic valve is located between the left ventricle and the aorta.

The atrioventricular valves are controlled by papillary muscles, which are specialised structures in the ventricles that attach to the valves via string-like extensions known as chordeae tendineae. They contract to prevent the atrioventricular valves from inverting and allowing blood to flow backwards into the atria.

There are three papillary muscles in the right ventricle which attach to the tricuspid valve:

1. Anterior
2. Posterior
3. Septal

Learn more about the anatomy of the right ventricle in this tutorial.