People have been using snowboards in the backcountry for some time. However, a snowboard can't go uphill. Backcountry snow boarders have used snowshoes or carried a short pair of skis for going uphill.
A splitboard is a special type of snowboard that solves this by separating into two sections that can be used like skis with special bindings that allow walking uphill.
For uphill mode
The board is separated into two sections by undoing a latch mechanism
The binding on each section is rotated to orient it along the board, allowing the boarder to walk uphill
Skins fitted to each section providing grip
For downhill mode:
The sections are latched securely together
The bindings rotated across the board.