More specifically, it's the serous pericardium, which is the inner layer of the pericardium. It's made of two layers:

* Parietal layer: This layer lines the fibrous pericardium (outer layer).

* Visceral layer: This layer adheres to the surface of the heart.

The space between the parietal and visceral layers is called the pericardial cavity, and it's filled with pericardial fluid. This fluid acts as a lubricant, reducing friction between the heart and the surrounding pericardium as the heart beats.