It's just as the name implies! It's simply water that has been blessed in some way or another. Typically, the water is imbued with the spiritual energy of the one performing the ritual, and it can be used to repel that which would be considered evil in the eyes of user.

To an ordinary person it would be no different from tap water -- but to something like a demon, the water would boil instantly on contact, burning their flesh and weakening them.