Different people have different traditions when it comes to paying their respects to the deceased. Some bring flowers to cemeteries while others place rocks upon the gravestones. There are also many who bring coins with them to graveyards. If you find yourself at a cemetery and see coins on someone's grave marker, don't touch them and certainly don't take them, they are there for a reason.
Most often seen on headstones of veterans, especially ones who died while serving in the military, the coins have different meanings depending on which is used, but they all are left as a message to the soldier's family that someone recently visited their loved one. If it is a penny, it signifies someone came and paid their respects. A nickel is left by someone who attended boot camp with the deceased, a dime is put there by someone who served in the armed services alongside them, and a quarter is placed by someone who was with the soldier when they died.
The tradition dates back to the Roman Empire and the money left on graves at national and state veterans cemeteries eventually is collected and used towards the upkeep of the cemetery and for paying burial costs for indigent veterans.