First, review the second link that Paul gave you.

Sections and section breaks are both straightforward and complex especially when you start using options on headers and footers.
Sections / Headers and Footers in Microsoft Word (Ribbon Versions)

If it says "First page Section 1" as a label for your footer, this means that you are on the first page of your book and you have headers and footers set for "different first page."
Header/Footer Settings Recap