VoiceOver is an Accessibility feature available on Apple platforms. It provides an interface between your app and your blind or low-vision users. In short, it's a screen reader.
Activating VoiceOver fundamentally changes the touch interface of an iOS device, so when you activate it, you need to know these basic commands to navigate:
The following video is a great summary. The content is succinct, high quality and up-to-date, even though the video quality is low and the demo is on an iPhone 3GS.
Settings > General > Accessibility > VoiceOver and toggle the top switch.
Alternatively, if you are activating/deactivating VoiceOver often, I'd advise to set it as the Accessibility Shortcut. When enabled, a triple tap on the Home button will toggle VoiceOver ON and OFF from anywhere.
Settings > General > Accessibility > Accessibility Shortcut