An ad replaces the "screen saver" when you unlock the device (until you swipe it away), and there's a small banner at the bottom of the book list in the main me

An ad replaces the "screen saver" when you unlock the device (until you swipe it away), and there's a small banner at the bottom of the book list in the main menu. You probably bought a more expensive version with no ads. They can also be removed later for a small fee, or you can simply never enable wi-fi and there will be no ads at all.