From Wikipedia:
“In Japan, Ksitigarbha, known as Jizō, or Ojizō-sama as he is respectfully known, is one of the most loved of all Japanese divinities. His statues are a common sight, especially by roadsides and in graveyards. Traditionally, he is seen as the guardian of children, particularly children who died before their parents. Since the 1980s, he has been worshiped as the guardian of the souls of mizuko, the souls of stillborn, miscarried or aborted fetuses, in the ritual of mizuko kuyō (水子供養, lit. offering to water children). In Japanese mythology, it is said that the souls of children who die before their parents are unable to cross the mythical Sanzu River on their way to the afterlife because they have not had the chance to accumulate enough good deeds and because they have made the parents suffer. It is believed that Jizō saves these souls from having to pile stones eternally on the bank of the river as penance, by hiding them from demons in his robe, and letting them hear mantras.
“Jizō statues are sometimes accompanied by a little pile of stones and pebbles, put there by people in the hope that it would shorten the time children have to suffer in the underworld. (The act is derived from the tradition of building stupas as an act of merit-making.) The statues can sometimes be seen wearing tiny children’s clothing or bibs, or with toys, put there by grieving parents to help their lost ones and hoping that Jizō would specially protect them. Sometimes the offerings are put there by parents to thank Jizō for saving their children from a serious illness. Jizō’s features are commonly made more babylike to resemble the children he protects.
“As he is seen as the saviour of souls who have to suffer in the underworld, his statues are common in cemeteries. He is also believed to be the protective deity of travelers, and roadside statues of Jizō are a common sight in Japan. Firefighters are also believed to be under the protection of Jizō.”