When something is arcane, only a small group of people know it or can follow it. It is not merely “secret,” like a password; it is knowledge that feels shut away because it is specialist, old-fashioned in form, or written so that outsiders cannot get in. Lawyers, programmers, and scholars use it for rules and details you only learn after a long time inside a field — tax law can be arcane, and so can the small print of a game. Though actually, people stretch it a lot: they call anything fancy and confusing “arcane,” even when it is only new, not hidden. In stories and games, Arcane often gets used as a name for magic, as if magic were a closed craft. That is a later, looser use. The word itself is rather formal; a student would more often say “only experts understand that.”