Sacred is the state of being holy, reserved for worship or associated with worship in a given culture or religion, as contrast with profane which is of the earth, daily, and common.
Every religion has both things and ideas that are scared, for example: the Torah, Bible and Qu'ran in the three Abrahamic religions, but also, places can be sacred, like churches, mountains where a prophet received his revelation, or cities where the religious leader died.
The concept of "sacred" can be applied to non-religious parts of a culture, if that culture would find them to be held above all else. In the United States, it could be argued that our the ideas contained in the Bill of Rights and Constitution are sacred to Americans and should not be changed lightly nor tread upon.
What is sacred to one religion or culture is not necessarily sacred to another, and may even be a profane idea. Pagans are supposed to have considered non-monogomous sexuality to be sacred. Christians found that concept to be a profanity against God's creation.