A flat or zero camber ski has no upward arch underfoot—instead of curving up like a cambered ski or curving down like a rocker ski, the midsection sits completely flat on the snow. Many flat skis still have a slight rise at the tip and tail to keep things catch-free.
This profile shines in the terrain park and for freestyle riding. Without camber pushing down into the snow, flat skis feel loose, surfy, and easy to press into butters and nose/tail presses. They're also forgiving on landings and make it simpler to pivot or slide sideways on rails and boxes.
The trade-off is edge hold and energy. Flat skis won't grip hardpack as confidently as cambered skis, and you lose that snappy rebound out of turns. If you spend most of your time lapping the park or jibbing around the mountain, flat camber is a great match. If you prefer carving groomers or charging steep terrain, you'll likely want some camber underfoot instead.