Active Frames are dynamic covers that are displayed when your application is minimized on the home screen. Every application uses application covers by default. Just swipe up from the bottom bezel within an app and the application cover is displayed automatically. Tapping the cover takes you back into the application.
The application cover is a non-interactive scene that the BlackBerry 10 SDK for Adobe AIR can automatically update whenever it changes. By default, the cover displays a scaled-down version of the scene that's currently being displayed in the application. For many applications, using the default cover might be the best option since it displays the current visual state of the app for the user.
However, there might be instances where you want to display specific information in the cover. For example, in a weather application you could display the current temperature. In AIR, you can create custom application covers using the QNXCover classes found in the qnx.system and qnx.events APIs.
For best practices for using application covers, see the Active Frames topic in the UI Guidelines.
For a more advanced sample of a dynamic active frame, check out the ActiveFrameSample sample on GitHub.