In modern Wicca and Paganism, when something is oathbound, it simply means that it is information which may not be revealed to people who have not been initiated into a particular tradition. What information is oathbound will vary from one tradition to the next. In Gardnerian Wicca, nearly everything is oathbound -- which is why if you see someone sharing information about the practices of a Gardnerian coven, you should be suspicious; they're either passing along information that is not Gardnerian, or they're someone who has broken their oath of secrecy. In other traditions, some practices are oathbound but others are not. Typically, the more emphasis a tradition places on initiations and lineage, the more information will be considered oathbound. Wicca, in its original form, was considered a "mystery religion," so there are some pieces of knowledge which are never meant to be shared with outsiders.
Examples: When Willow was asked by a reporter about the practices of her coven, she was unable to reveal the information, because it was oathbound material.