Rocker is the bottom curve of a surfboard; the rocker will carve a particular arc when leaned on for a turn. A full rocker will have a tight sharp turn where as a flatter rocker will have a more drawn out turn. The rocker of your board could be a smooth continuous curve or built so that a particular part of the rocker is engaged with the wave when you're standing on the “sweet spot” of the board.

    Flatter rocker is faster, paddles easier and catches waves earlier, but makes a board more difficult to turn. Fuller rocker is much slower, paddles worse and catches waves later but turns on a dime. If you're in hollow surf you would want more rocker to avoid pearling (nose diving) and if you're in mushy surf you would want less rocker to help you get through the flat sections.

Comments:
    Short boards can get away with less rocker because the thin nose will actually flex on faster, more hollow waves where you will need the extra rocker. If you notice that you are having problems dropping into a wave, this could be because the wind coming up the face of the wave is getting caught under the nose of your board. You may want to get a board with less rocker to avoid this problem.