In religion, the sacred are concrete objects or places in the real world that individuals of that faith consider to be symbols of their religion, and therefore Earthly reflections of the supernatural components of their religion. I will assume in the context of your last question that you are referring to the sacred as Durkheim saw it.
Durkheim saw the sacred as the components of a religion that were separate from the supernatural elements of the religion. He drew on the example of totemism as the fundamental basis of the simplest form of religion. A totem is any object or living creature (plant or animal) that a group considers representative of themselves, and is therefore sacred. This reverence of the sacred Durkheim viewed as the cohesive element in a society, further stating that this cohesion provided by the sacred was the function of religion and a reflection of a society's concern for itself.