The term means either increasing or decreasing the amount of movement on given control surface.

For example if your elevator has 1/2 up and down movement from neutral then making the surface move 3/4 would be increasing the throw and making it move 1/4 would be decreasing it.

It can be done mechanically by moving the pushrod in or out on the control horn or on the servo arm.
(moving the clevis to a hole closer to the control surface on the control horn increases the throw moving it away decreases it. On the servo side the opposite is true moving the pushrod close to the servo spline increase throw moving it away decreases it)

Some radios have end point adjustment and or a dual rate setting that allow you do it electronically.