Passbook is a central place to store things like boarding passes and loyalty cards in one place on your phone. You can get passes from apps, email, or websites. As an example, if you download the Walgreens app, you can sign up for (or sign in to) your walgreens loyalty account and then add the pass to Passbook. You get to pick your favorite store and then whenever you are there, the pass is ready to use on the lock screen.
Another easy example is to visit http://www.coupons.com from your iPhone. You can add coupons directly to Passbook from the website.
For more information, see http://support.apple.com/kb/HT5483. Hope that helps.