In layman terms, a zoom in.

Given the screen size and resolution, UI controls such as buttons, edit boxes, menu and description text etc would be unreadable. For example you would end up with 1mm high text etc. So everything is scaled up 2x, to make it usable. But the drawback is that it effectively halves the screen resolution.

EDIT:
The alternative would be to create a completely different UI, designed specifically for mobiles. With 1:1 pixel mapping but where buttons and text would be ridiculously large if displayed on a big screen. But that is not what Purism is doing at this stage. They want to shoehorn a desktop UI into a small screen. Which makes scaling the only option.