The sleep/wake button allows you to turn the screen off (put the device to sleep) and turn the screen back on (wake the device). On a non-dedicated device, the button also allows you to power the device on and off.
To power on a non-dedicated device: Press and hold the sleep/wake button until the Apple® logo appears on the screen.
To power off a non-dedicated device: Press and hold the sleep/wake button and the volume up or volume down button until you see the Power Off screen. Then slide the onscreen power off button to the right to turn the device off.
Sleep Mode
On a dedicated or non-dedicated device, press and release the sleep/wake button once to turn the screen on (wake) and turn the screen off (sleep).