By default an iOS app will support all possible orientations (landscape, portrait left, portrait right, etc.). When a user rotates the device the app will automatically rotate to fill the screen. In many cases this may not be a desirable behavior. For example, your app may have been designed for portrait mode only. In more complex cases you may have to support different orientations for different screens of the app. For example, a video player must work in either portrait or landscape mode where as the rest of the app only supports portrait.
The process to control how the app behaves when the screen is rotated can be a bit confusing. This article will shed some light on the issue.