It's a method of prayer where you enter from the front gate of a shrine or temple and make your way towards the haiden or hondou to pay your respects, before returning to the entrance and repeating the same for a hundred times in hopes of getting your wish fulfilled. Well, it's not like you will actually get your wish if you do it though. Haiden (lit. "worship hall") refers to the front part of a Shintō shrine (that same place Reimu keeps her donation box) while Hondou (lit. "main hall") refers to the main building in Buddhist temples. Worshippers usually offer their prayers in these places when they go to a shrine or temple.