Cutoff and phase control are used in order to regulate the power supply to your lights, in order to be able to use a dimmer.
Cutoff: In this method, a portion of the alternating current wave is cut off, reducing the power delivered to your lights. This is typically used on incandescent light bulbs.
Phase control: In this case, the beginning (phase) of the alternating current wave is cut off, removing a portion of the wave. This is typically used for inductive or capacitive loads, such as when dimming LED lights.
Both techniques are used to adjust the brightness of lighting and to save energy.
What to look out for?
It is very important to check what types of lamps you are using and what technique to applicate. A lamp that has to be dimmed with a cutoff technique will not be dimmable by phase control and vice versa. Fortunately, nothing of serious damage can occur, but it will simply not dim your lights.