3D Accelerometer Rotation is a feature that allows flexibility in how the content is viewed on the screen. Simply turn the device from vertical to horizontal (or horizontal to vertical). The device senses the change in orientation and automatically adjusts to fit the screen.
Camera, album, music player, videos, web browser and many other apps take advantage of this feature.
From a Home screen, tap Apps (located at the bottom).
From the DEVICE section, tap Display, gestures & buttons.
Tap Auto-rotate screen to enable or disable.
Enabled when a check mark is present.