Cnote,
> Could somebody please explain? <
Expansion is the opposite of compression. With a compressor, when the signal rises above a threshold the level is reduced automatically. So the overall dynamic range is reduced. An expander does the opposite, either processing loud parts and making them louder still, or making soft parts even softer. In the case of "downward" expansion, that means the device is making the soft parts even softer.
A noise gate is an extreme case of downward expansion because it takes quiet stuff and turns it completely off.
--Ethan