In Greek Mythology Charon, the ferryman of Hades, requires payment of one coin (called Charon’s Obol) to ferry your loved ones soul across the River Styx that separates the living from the dead. Historically, the coins were placed in the mouths of the deceased, or according to some sources, over their eyes.
In America’s military a coin left on a headstone or at the grave site is meant as a message to the deceased soldier’s family that someone else has visited the grave to pay respect. Leaving a penny at the grave means simply that you visited. A nickel indicates that you and the deceased trained at boot camp together, while a dime means you served with him in some capacity. By leaving a quarter at the grave, you are telling the family that you were with the soldier when he was killed.