"viewports" are just a new feature internal to the Rockbox LCD drivers that allow other parts of Rockbox to draw in self-contained rectangles (i.e. things like scrolling and colours only apply within that rectangle), rather than the entire screen.
The "list viewports" change that was committed recently was just an change to how the "list widget" (which is used by the menus and file/database browsers) works internally - splitting the list into separate components (title, icons, scrollbar list items etc), each drawn in its own viewport.
The intention is that viewports is going to allow better screen layouts, with more user-configurability, but currently it's just an internal thing.