A Page Library contains the pages from one or more of your documents, providing you with a selection to choose from when creating something new.
Example scenario
Star Recruitment have created different versions of their monthly newsletter (they like to keep things interesting with a mix of page designs and interactivity!). However, they would like all of the pages in one place so they can pick and choose which ones to use next time they create a newsletter.
Solution!
The solution is to create a Page Library and add each version to it. This means all the pages from each will be available to them when they next create a newsletter - all they need to do is select the 'Page Library' at the creation stage!
Read the Page Libraries Guide - https://view.pagetiger.com/TigerLabs/page-libraries