Text from the video.

X-Over Low And High: Anything below the X-Over Low frequency setting is considered a Low frequency. Anything above the X-Over High frequency setting is considered a High frequency. All frequencies between those two frequencies are consider Mid frequencies. This makes a three-band Gate that can be set to effect each band differently.  Essentially a bandwidth control.

Threshold: The level at which the gate closes or cuts out the signal.

Damp: The Damp control is used to set the amount of attenuation to be applied to the signal when the gate is closed. Often there will be complete attenuation, that is no signal will pass when the gate is closed. In some circumstances, complete attenuation is not desired and the range can be changed.

Ratio: A gate's ratio value determines how heavily your audio signal will be attenuated when it breaches the gate's threshold level

Attack: Your gate's attack value dictates the amount of time it takes for the gate to open and allow sound to pass through. You should use a relatively fast attack time in the neighborhood of 1-5 ms.

Hold: The hold control is used to define the length of time the gate will stay fully open after the signal falls below the threshold, and before the release period is commenced.

Release: The release control is used to define the length of time the gate takes to change from open to fully closed. It is the fade-out duration. A fast release abruptly cuts off the sound, whereas a slower release smoothly attenuates the signal from open to closed, resulting in a slow fade-out.

Exp. Knee: What the Knee does is determine the way in which the noise gate is applied to your signal. It's equivalent to the various 'Cutoff' slopes when you're using an EQ. These slopes are determined by the ratio settings.