Something real is genuine. If you inherit some jewelry, you'll have to take it to an expert to know if those diamonds are real or just cubic zirconia, which looks like a diamond but isn't worth nearly as much.
That seven footed monster your big brother used to tell you frightening stories about isn't real, and neither, sorry to say, is Santa Claus, because to be real, something has to actually exist. Real has roots in the Latin word res, meaning "thing." We also use real to mean "significant," like when we say something is a real problem, or when we say we're really, truly sorry about something. In old Spain or Brazil, a real is a coin.