This skill is the ability to recognize the trappings of the occult and deduce its purpose, or to create some trappings yourself. We don't judge. It can be as obvious as, "Hmm, the room of mirrors with a pentagram in the middle and bloodstains might not be quite right" to, "Neat, this scribble is the true name of the Duke of Hell, Galorigan". All occult knowledge would fall under the ban on magic in the laws of the Compact, and it's a perennial favorite of the Inquisition to describe in questionable detail the supposed practices of the accused who are no longer in a position to refute it.
For information about what your character should generally know if you have this skill, do 'help occult 1', 'help occult 2', etc.