The word occult comes from Latin occultus (hidden), referring to supposed secret or hidden knowledge usually of the supernatural. Sometimes the knowledge does not consist of access to particular facts, but is arrived through the mind being taken through a process triggered by a text, image or performance. See occultism.
Many (especially conservative Christians) use it to refer many practices which they disapprove of but which those who participate in it do not consider occult (Dungeons & Dragons, heavy metal music, even sometimes Catholicism).
Even for those who do consider what they are doing occult (e.g. ceremonial magicianss, Satanists, etc.), their beliefs and practices are normally far from being secret or hidden, being found very easily (in books or on the Internet).