The structure indicated is the coronary sinus.
The coronary sinus lies posteriorly on the heart in the coronary sulcus and receives four cardiac veins:
- Great cardiac vein
- Middle cardiac vein
- Small cardiac vein
- Posterior cardiac veins
The coronary sinus returns deoxygenated blood to the right atrium of the heart, together with the superior and inferior vena cavae.
Learn more about the anatomy of the venous drainage of the heart in this tutorial.