Actually, it means from left to right and such. For example:
(book1) (book3) (book5) (book6) (book2)
So, Because there are six books to arrange on the shelf, there are six stages in the problem. You must decide what the first book will be, what the second book will be, what the third book will be, and so on.
There are 6 choicses for the first book, 5 choices for the second book, 4 choices for the third book, and so on.
6 x 5 x 4 x 3 x 2 x 1 = ?
= !6
=720
Six books can be arranged 720 different ways.