These are applications that are included on a device when it is purchased from the manufacturer or carrier. These apps are already present on the device before the user ever turns it on for the first time. For example, a new smartphone might include apps for email, web browsing, maps, and a suite of tools designed by the device manufacturer.
The inclusion of these applications is significant for several reasons. For manufacturers, they offer an opportunity to differentiate their devices and provide a specific user experience out of the box. They can also serve as a source of revenue, through pre-arranged agreements with application developers. From a historical context, the practice has become increasingly common as mobile operating systems have matured and the app ecosystem has expanded.