Previously Windows Phone's screen rotation was always automatic (controlled by the accelerometer sensor). Being able to lock the phone to a given orientation is useful for larger screen devices (e.g. when being held at an angle), or when consuming certain types of media (e.g. browsing the web).
Video description:
After installing the Windows Phone 8 Update 3 (GDR3) a new setting is added to control automatic screen rotation. This makes it possible to lock the screen is a specific rotation orientation.
More on Windows Phone 8 Update 3 (GDR3)
We covered the key new features and updates added by Windows Phone 8 Update 3 (GDR3) in our original news story. The update will start being delivered over the air to existing devices next month. However, developers and interested consumers can get early access to the update through the Windows Phone Developer Preview Program.
In terms of the consumer roll out of the update HTC and Samsung devices are likely to get Windows Phone 8 Update 3 first. Nokia's Lumia devices will get the update a short time later, a result of Nokia's plan to include a number of its own updates and enhancements alongside the Microsoft update (i.e. as with the Lumia Amber update, Nokia will be delivering a significant firmware update on top of the Microsoft update).
See also
Video: Driving Mode in Windows Phone 8 Update 3