Dry bags keep clothes and equipment dry. There are many models: the thinnest work inside a backpack or side bag where the thicker ones, the so-called canoe sacks, are made of tarpaulin-like and very durable material. You can tie one to the cockpit or the stutter on the bow of the kayak.
The mouths of the dry bags and canoe bags are wrapped around the plastic strip at the mouth. The lap recommendation varies a bit, but 3 to 5 laps are the most common.