Device is usually shorthand for "hardware device" like a desktop computer, laptop, smartphone, or tablet. The key differences for portability purposes are the interface (mouse, touch screen, etc.), the performance (CPU or GPU power), screen orientations, and screen size.
Why is portability needed for devices?
Due to varying performance qualities, screen sizes, screen orientations, etc. that differ from a desktop, content must be able to adapt to all of these devices. These devices do not only change the way a device is being used, but enhances the quality and appearance of the content as well.
How does FANDOM handle content portability?
FANDOM Development staff and a volunteering group (also known as Vanguard) assist with content portability on FANDOM. These users allow content to be portable on devices by manipulating code, creating portable infoboxes, working with the mobile main page, etc.