The book provides instructions for the 'player' and a ledger to record their activity. The idea is that when you open the book, you become The Stork. You now have 24 hours to deliver your package. Your package is either tangible or intangible, but generally involves something useful, something you have made, and/or a positive interaction. The book provides guidelines for choosing an appropriate package and recipient. Giving someone art you've made, asking someone out on a date, giving away a concert ticket, or finally apologizing are all examples of packages one might deliver.
When you deliver your package, you pass along the book as well. The recipient becomes the new Stork and they get to deliver a package of their own. The ledger in the book tracks the line of Storks and the packages that have been delivered.
The idea is to encourage people to be brave and kind and to share themselves and notice the impact they have on the world.