If you continued in math, you learned about i , the square root of minus one. It does not exist and can not be changed to a real number. But its concept is handy to use in finding other numbers. For those of you who don't like math, let's take a look at literature. There is fiction, which means unreal , and there is non-fiction, which means real.
Fiction is a story that never was real.
Non-fiction is a story that once was real. Then there are pictures. Let's compare art with photographs . A picture painted by an artist is that artist's unreal conception. It may be of a real subject, but it is nonetheless the artist's conception, or unreal idea, of what it looks like.