The heart is covered by the pericardium, which is made of two layers. The layer in direct contact with the outside surface of the heart is the epicardium. It's a thin, glossy membrane. The outer layer of the pericardium is slightly thicker and attaches to several areas in the chest to anchor the heart in place.
The heart rests inside the sac created by the two layers. A small amount of fluid resides between the layers directly over the heart. This fluid acts as a lubricant, allowing the heart to beat with minimal friction.
Another smooth, glossy membrane, called the endocardium, covers the inside surfaces of the four chambers of the heart, plus the valves and the muscles that attach to the valves.