Pages are a type of collectible item that you can exchange for other in-game rewards. You can earn them by completing tasks and winning events, buy them in bundles,  and even spot them at Hogwarts itself, just laying around. Pages can then be converted into Notebooks or used to bring back failed Special Adventures. 

How do I convert Pages into Notebooks?
You can convert Pages into Notebooks and vice versa by selecting the "Pages Exchange" option in the Library. This option unlocks in Chapter 4 of Year 3 and you can use it once per day.
 

How do I use Pages to replay a missed Special Adventure?
You can replay a missed Special Adventure by spending Pages and unlocking it in your Memory Book. To access the Memory Book, tap on a polaroid photo icon at the bottom-left part of the screen. Then choose the "Adventures" section to see the list of your available quests, as well as their cost in Pages. You may attempt any Special Adventure that you qualify for and haven’t finished as many times as you like, however some Adventures may have a cooldown timer that must run down before they may be attempted again. Successfully completed Special Adventures will not appear in your Memory Book.
 
 
PLEASE NOTE: While some failed Adventures appear in the Memory Book immidiately, others might take some time to become available. We are actively updating the Book with new content, so more Adventures will be added there with future updates to the game. However, Quidditch Adventures will not appear in the Memory Book, as players get only one chance to complete them.