He meant "collective".
You have to adjust the collective (which is usually assigned to the throttle axis of your joystick) when your speed change.
The rotor acts as a wing. When your copter is going fast, the rotor creates a lot of lift, so you don't need much collective to maintain a level flight (= stable altitude).
However, when the speed of your copter decreases, the lift decreases as well, so you have to compensate by increasing the collective, so that the rotor creates more lift again.
So that means that if you slow down and do not increase the collective, the copter wil start loosing altitude. In the same way, if you were going slow and suddenty you accelerate, and you do not decrease the collective, you will notice that the altitude will start to increase.
To maintain the helicopter stopped in mid air, (that means "hovering"), you have first to learn to fly slowly, then slower and slower until you get able to control it when it's stopped. The slower the speed of a copter, the harder it gets to control it. You have to TAKE YOUR TIME ! Just practise
