Page one of any book is when you first open a book, it is on the right hand side.
When you turn that page, page two is on the left, and page 3 is on the right.
If you use the page numbering template, you can start your page one, on whatever page you want. A lot of times, people may number a book, and they won't number the cover page (Page one in Book Bolt). And or your book may have a table of contents as well and they don't want that numbered). So if your cover page was on page one, and a table of contents on page two in book bolt, you may want to start your page numbers on Page 3 in Book Bolt.
So Page 1 in the book, would start at Page 3 in Book Bolt.