What is the meaning of the stones or coins placed on a headstone?

Within the military, there is a tradition that leaving coins on headstones of deceased service members is a way of showing respect. Different coins mean different things. A penny means you know someone who was associated with the deceased (relative). A nickel means they trained with the deceased. A dime says they served with the deceased. A quarter means they were present or assigned with the deceased when they lost their life. The tradition of leaving coins is not limited to the military; it has become a way for anyone to show that they were thinking of someone, which is why you will see coins left on graves of non-military members as well. Stones or rocks are left on headstone to indicate the same. It is believed to have originated as a Jewish custom and indicates respect for the dead.

C. A. L. L

I couldn't gave said it any better

Showing respect & letting the loved ones know these heroes are not forgotten.