Rocker refers to how much curve is along the bottom of a kayak from its front to back. Speaking technically, rocker refers to the curvature of the bottom of the hull from the bow to stern. The more curve it has, the more rocker it has. Or in other words, when a kayak has a lot of rocker it is very curved along the bottom. A kayak with little or no rocker will be flat on the bottom.
A lot of rocker makes a kayak easy to maneuver which means it is easy to turn. However, a kayak with a lot of rocker doesn’t track well because the front and rear of the kayak do not have a lot of contact with the water. A kayak with no rocker would track really well but you would have limited maneuvering and some rocker is necessary for navigating / handling rough water. Kayaks designed for flat water will have less rocker than kayaks designed for whitewater which require easy maneuverability. The rocker of a kayak can be symmetrical between the bow and stern or there can be more rocker in the front than the back. For example, some kayaks have more rocker in the bow than the stern, which allows the paddler to turn the front of the kayak while the stern has less rocker so that more of the kayak is in contact with the water helping it to track better. On the flipside of this, many whitewater boats have a symmetrical rocker because tracking well is less important and maneuverability is critical. With the exception of whitewater kayaks, rocker design needs to be a balance or compromise between providing sufficient tracking and also maneuverability.
In practical terms, when you are in a kayak with a lot of rocker it is easy to turn but won’t track well. (When you paddle on the right the kayak will have a tendency to go left and when you paddle on the left the kayak will have a tendency to go right.) When you are in a kayak with little to no rocker it should track really well but it would be hard to turn.
Sources:
https://www.austinkayak.com/blog/2012/08/a-look-at-kayak-hull-designs/
https://cdkayak.com/kayakdesign.aspx
https://kayakguru.com/guide-to-kayak-hull-designs/
https://www.austinkayak.com/blog/2012/08/a-look-at-kayak-hull-designs/