A coin left on a grave means that someone has visited the grave site to pay their respects.
Often, for military service members, the type of coin has a special significance:
Quarters mean that the person who left it there was with the fallen soldier when they died.
Dimes mean that the two served together at some point in their careers.
Nickels mean that the person who left it there trained with the deceased.
Pennies mean that a fellow serviceman or woman has recently stopped by to pay their respects, whether or not they knew them personally.
Read more: Leaving Coins on a Grave - Meaning & History