In Sweet Magnolias, collectibles serve as a scrapbook of your journey through the story. They reflect important in-game decisions, the relationships established with the different characters, and the endings you've achieved.
Some collectibles are unlocked as you complete chapters, but to get them all, you'll have to replay the story multiple times and explore its various paths. Don't worry, hints will guide you to get them all.
You can view your collectibles while playing Sweet Magnolias by tapping the square icon next to the menu icon.