A ritual is a ceremony or action performed in a customary way. Your family might have a Saturday night ritual of eating a big spaghetti dinner and then taking a long walk to the ice cream shop.
As an adjective, ritual means "conforming to religious rites," which are the sacred, customary ways of celebrating a religion or culture. Different communities have different ritual practices, like meditation in Buddhism, or baptism in Christianity. We also call the ceremony itself a ritual. Although it comes from religious ceremonies, ritual can also be used for any time-honored tradition, like the Superbowl, or Mardi Gras, or Sunday morning pancake breakfast.