Traditionally, he is known to be the guardian of children, particularly children who died before their parents. Since the 1980s, the tendency developed in which he was worshiped as the guardian of the souls of "mizuko", the souls of stillborn, miscarried or aborted fetuses.

In Japanese mythology, it is said that souls of children who die before parents are unable to cross the mythical Sanzu River on its 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 usually 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 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 also commonly made more babylike in order to resemble the children he protects.

As he is the saviour of souls who have to suffer in the underworld, his statues are common in cemeteries

-- from Wikipedia --

This shot was taken at Hasedera in Kamakura, Japan.