This is new as of 3.4
Its another means of getting loot to put it simply, there are 4 goals to aim for, 1st is get 9 stickers by doing the content in the book, then getting those stickers to line up into 1-3 lines (3 lines to get all rewards)
Its simple, just get the book in Idylshire open it up to see what content you can choose from, there will be 16 tasks, you can only do 9 for stickers, but you can always get second chance points.
When you complete a task you go back into the book and click the task to get your sticker which is determined randomly. To control where the stickers are to a tiny degree you use your second chance points (earned when you do a duty with a new person). You can only do that when you have 3-7 stickers.
Now when you complete content it doesn't count the sticker until you click the task, so if you have your 9 tasks done, click 7, use the second chance points to shuffle until you find a pattern that requires 2 stickers to get 3 lines or you run out. If you run out you can farm more second chance points, there are lots of parties at the time of this post in the Party Finder.
You can get i250 Gear with 5 materia slots, minions, tomes, MPG, etc. Good luck!