When you are shopping for tickets, you are "holding" real inventory. No one else can purchase those tickets unless you release them by not completing your order. Due to high demand, they've implemented a time limit during the checkout process. Each checkout page is assigned a different time limit based on the type of information they need from you. If you exceed the posted time on the page, the tickets you are holding are released for others to purchase. You have about five minutes total to complete your purchase.