Placing a small stone on top of a headstone is a Jewish custom demonstrating respect for the deceased and signaling that there has been a visitor to the grave. Jews call these stones of remembrance.
At a Jewish burial it is customary for each mourner to add a stone during the Jewish ceremony Mitzvah of Matzevah (setting a stone). Later visitors to the grave add stones to the headstone or monument to commemorate the life of the deceased and to demonstrate that they continue to build the monument. This custom is illustrated at the end of the movie “Schindler’s List” when the individuals saved from the Holocaust placed stones on Oskar Schindler's monument as they walked past.