As you rub the wool on the board the ribbed surface accelerates the felting process.
Curved grooves allow soapy water to flow as you rub.
Try making a ball to get a feel for felting:
Take a handful of carded wool, pour over it a mixture of hot water (as hot as you can handle) and detergent. Soap powder can be used instead of detergent, but dissolve the powder well in the hot water first. Rub the wet wool between you hands to form a ball, and continue squeezing and rubbing until you have a solid ball of felt. It is surprising how much wool goes into a small ball. Experiment with layers of different colors of wool. Completely cover the entire ball with each felted color, before adding the next layer. The ball can be cut to expose these layers. Rinse the ball in cold water, then hang out to dry.