"Rates" actually refers to the total amount of movement that is available from a control surface (rudder, elevator, aileron). Low rates would mean that with the control stick on your transmitter moved to its limit, the appropriate control surface moves a small amount. Smaller movements of the surfaces mean aerodynamic force is available to make whatever maneuver you are trying to do happen. Maneuvers are slower and gentler. Higher rates mean that the control surfaces move more for the same input from the transmitter. Maneuvers are executed more quickly and more advanced maneuvers become possible.

The term dual or triple rates means that by flipping a switch on your transmitter you can change from low rates to high and possibly even one setting between while flying. You may want low rates for take-offs, landings and smooth high-speed aerobatics, mid rates for more aggressive aerobatics like snaps and spins, and high rates for 3D stuff like hovers, harriers, blenders and stuff.

Different types of flying require different "rate" settings. I've seen pylon racers with so little control surface movement that you'd swear they had none, and I've seen 3D planes that wag their tails like a happy puppy when hovering.

If you have a relatively fast Internet connection, go to scaleaerobatics.com and have a look at some of the videos from the 2002 Tournament of Champions, if you watch the airplane, you can see a TON of movement on the control surfaces during slow maneuvers.