Mizuko kuyō is a special ceremony that happens in parts of Japan when a baby doesn't make it all the way to being born, or passes away soon after being born.
In Japan, parents often believe that all babies who pass away early become special spirits called "mizuko," which means "water babies." These spirits are believed to still need their parents' help to find peace in the afterlife.
So in a mizuko kuyō ceremony, parents and loved ones get together to remember the baby and offer gifts like flowers, toys, or sweets to the mizuko. They might also bow or pray to a shrine, which is a special place for spiritual worship in Japan. The point of all of this is to show respect for the mizuko and help them find peace so that both the spirit and family can feel better.
It's a very sad occasion, but helps bring closure and comfort to the parents and loved ones who are grieving.