The Arcana can refer to one of two things - the symbols depicted on the trump cards used in tarot, or the supposed "cosmic archetypes" who can be contacted through the cards. The "cosmic archetypes" existence is up for debate in-universe. Most people do not believe in them, and likewise think that the fortune-telling card readers who claim they are real are frauds.
There are 22 Major Arcana in a typical tarot deck. Only one, the Magician, is a real being. The others are archetypes invented by the Magician, who wanted to make up his own religion with himself as the central deity.