First sworn by the wizard Bonisagus, the Code of Hermes formed the basis for the Order of Hermes. Also referred to as the Hermetic Oath, all members must recite and swear to it before they are admitted into the Order. The Code of Hermes ensures the continued existence and protection of the Order and its members. Simply put, they are the rules that Hermetic magi must live by.
The rules include things such as prohibiting using magic to scry on other members of the order, interfering in the affairs of the mundane world, or attempting to slay other members of the Order. These are all things that had brought previous magical societies to ruin and thus the provisions of the Code of Hermes are strictly enforced for the good of the Hermetic Order.