Rocker is the curvature of the board. If you set it on the ground you can see the amount of rocker it has by how much the tip and tail sit off the ground. Continuous is a continuous curved flow whereas a 3 stage has more of an abrupt curve.
A continuous rocker board has a smoother, more predictable pop while the 3 stage has a more bucking, unpredictable pop. As a beginner it's usually good to be on a continuous rocker board and once you get better you can demo both types to see which one works for you. I have come to like a three stage board because I try to ride w2w as much as possible while getting the most height possible. If I were a person who did a lot of moves into the flats I would probably prefer a continuous.