On 6CH radio controlled helicopters (and full-scale helis for that matter), the main blades can change their pitch angle. What this means is that if you sit the heli on a table and look at the tip of one of the main blades, the chordline of the blade can be tilted through a range of angles by the servos. In this sense, the rotor disk of a heli is a bit like a variable-pitch prop on an airplane. If the heli is hovering and you wish to make it climb straight up, you increase the pitch of the main blades, and increase the throttle so that the engine can overcome the increased drag and keep the blades turning at the same speed. The increased blade pitch results in more lift, and so the heli climbs. (With R/C helis, unlike R/C airplanes, engine RPM's are supposed to stay the same over (most of) the throttle range. At high throttle the engine puts out more power, but there is a corresponding increase in the load on the engine due to increased main rotor blade pitch, and so the engine stays at the same RPM's.) This overall increase in pitch that makes the heli climb is called collective control.

So basically on a 4ch helicopter the angle of the blades is fixed but on a 6ch helicopter the angle of the blades changes to give more lift and control.