Why would anyone worship a rock or a tree? What’s up with those schoolgirls wearing white and red, and sweeping all the time? What are those weird charms with Japanese written all over them? I’ve thought these questions over myself from time to time and discovered that for many of them the answer is Shintoism (神道), a polytheistic religion that originated in Japan.
Shintoism is a collection of methods and rituals to improve human relations and ensure that there are no problems in the coexistence of human beings and nature. Life and death are not concerns of Shinto, instead finding happiness in this life is more important. Respecting family members is also another key point of Shinto. According to Shinto, nature is sacred. When we are in contact with nature, we are closer to the gods. hence that’s why many trees and rocks are considered sacred. Supposedly gods are living inside of them. Under sacred trees, people often hang ema(絵馬), wooden plaques with their wishes written on them for the god to make come true. If your wish is granted, you must return and hang another ema to show your gratitude to the god.
Many popular anime feature a Japanese highschool (or college) girl who is working at a Shinto temple while wearing white and red and often sweeping up the place. These girls are working as miko (巫女) or shrine maidens. Girls who work there part-time and make a modest wage assisting with shrine functions, performing ceremonial dances, offering fortune telling (also known as omikuji 御御籤), and selling souvenirs (like omamori 御守which are charms that provide protection and/or luck). Some of the more popular miko in anime include Sailor Mars from Sailor Moon, the Hiiragi sisters from Lucky Star, Kagome Higurashi from Inuyasha, and Miaka Yūki from Fushigi Yûgi. A more complete list can be found on TV Tropes.
There will be more posts about Shintoism in the future, but this gives you a very basic idea of what the religion is about and answers why so many highschool girls seem to be at the temple (at least it bothered me for a while).