Camber refers to the curve of the shape of a snowboard. Rocker (also known as reverse camber) is a curve opposite to that of of traditional camber. A board with rocker placed on a flat surface will make contact with the surface in the middle of the board while the the board will gradually rise from the surface the farther you get from the centre. Think of a banana.
Rocker reduces the amount of edge contact during turns, so it's not well suited for riding high-speeds or hard carving. Rocker helps a snowboard to float in powder and makes it easier to press and butter.