Basically, it's the opposite of camber. Alot of traditional skis have camber, which means when no pressure is put on the ski, the ski is higher in the middle than the tip and tail, like so.
Rocker or early rise or reverse camber are when the skis contact point with the snow is closer to the boot or binding or further away from the tip or tail. Rocker or reverse camber are usually closer to the opposite of camber, meaning they have no arc under foot, and the only point of the ski touching the ground is where the boot is. Skis like this are normally only used in powder, since you have less control on the hardpack and groomers because of the low amount of surface area in contact with the snow. Examples of these types of skis would be K2 Hellbents and Armada ARG. Early Rise is usually talking about a ski with camber underfoot and small amounts of rocker in the tip and tail. Skis like this are usually alot more versatile, as the rocker aids in flotation in powder, but the camber still allows an edge on groomers. Examples of these types of skis would be Salomon 2012s, Armada Halo's and Salomon Shoguns.
Example of rocker
Hope that helps!