How to understand glOrtho function?

I am the beginner, and i am reading the book named <<OpenGL Super Bible>> (3rd edition), when i turn to page33, there is a glOrtho function, i can not understand it, may u help me, thanks.

I don’t have this book.
Reading this :
What are the parts you have problem with ?
Basicaly glOrtho define a “rectangular cuboid” to be the visible volume, and fits it in the glViewport.

Thanks a lot.