I need help with the definitions of these type of characters.?

I'm a film student and my teacher told us to create 4 characters and the types are: shadow, threshold guardian, shape shifter, and heralds. Can someone please define what they mean? I don't know what these words mean and he doesnt really tell us. LIke I know shadow is like evil but i still dont know what types of characters they need to be in..and maybe give me an example from movies that have those characters in them? Thanks you!!!