An hard gate is an easy but quite interesting tool (for high gain rigs).
Basically you have 2 parameters:
1) a "close" threshold (in dB)
2) an "open" threshold (in dB)
When the input signal (coming from the guitar) become lower than the close threshold, the gate "close" and no signal will come out of it until the input signal will become higher than the open threshold.
It's usefull to mute the guitar during a song when you need 5 second of rest and don't want to roll the volume pot.
So, strike your power chord, mute the string, the gate will close, wait 5 seconds or whatever, strike the chord again and the gate will open.
Point is that during that 5 seconds of pause, your guitar is totally muted, no hum, no noise, no feedback.
Without the hard gate, you have to rely on rolling the volume pot in order to stay silent...but if you have to do it relatively fast, it's quite annoying (think of the intro to Metallica Damage inc when guitars kick in, to have an idea of what I'm talking about).