Any person or thing that's mysterious, mystifying, hard to read, or impossible to interpret is inscrutable. You ever notice how it's hard to tell what some people are thinking? Those folks are inscrutable.
A good way to think about what inscrutable means is to consider cats and dogs. Dogs wear their hearts on their sleeves, shaking when they're afraid and bouncing up and down when they're happy. Dogs are definitely not inscrutable, because you can tell what they're thinking and feeling. On the other hand, cats are very difficult to read. Even longtime cat owners aren't always sure what's going on with their kitty. Cats are very inscrutable animals.