Anytime you purchase anything from the App Store, iTunes Store, or iBooks Store, a 15 minute window is activated during which no password is needed to purchase anything. This window can be turned off completely to prevent unintentional splurges, or for security reasons. To do so, go to Settings -> General -> Restrictions. Set up a restrictions password if you haven’t already, navigate down to Require Password, and select your preferred option.
Have fun with your iPad!