Original: If used, the spraydeck or similar waterproof garment attaches securely to the edges of the cockpit, preventing the entry of water from waves or spray, and making it possible, in some styles of boat, to roll the kayak upright again without it filling with water or ejecting the passenger.
Simplified: The spray skirt or similar waterproof garment attaches securely to the edges of the cockpit, that no water from waves or spray may enter the boat.