SKULLS AND BONES ABOUT IT
Most pirate flags show skulls and crossbones or skeletons, meant to warn of death if the victim did not surrender. Many of these symbols were probably borrowed from gravestones of the time. A few of the flags also contain an hour glass, meaning that time is running out for the victim to surrender peacefully. The pirates wanted to project fearlessness in the face of death, and some flags pictured the captain toasting, dancing with, or literally conquering the skeletal dead. Hearts to were often pierced to symbolize "no mercy".
In an era where symbolism in art and everyday life was commonplace, each symbol had a distinct and immediately recognizable meaning. Also pirate flags were a very important form of psychological warfare, especially when combined with a reputation of not showing any mercy if opposed. If pirates could intimidate an enemy to heave-to and surrender without offering any resistance, then the harm they faced would be greatly reduced or possibly eliminated.