Compressor / Gates
Some compressors come equipped with a built in gate. A gate is a great way to isolate the important frequencies of a recording while getting rid of unwanted bleed that may have occurred while tracking.
When you compress something, you are, on the one hand, taming the dynamic peaks of the performance, but are also bringing up the noise floor and background noise of the room you tracked in. This can become a serious problem with an instrument like drums using multiple microphones picking up every sonic aspect of the kit. For example, if you want to compress the snare drum for more depth, you will also start making the hi-hats and cymbals louder in that microphone, making it hard to dial in on the key frequencies of just the snare drum.
With a gate, you can set a threshold that only allows the transients of the snare drum to open the gate, eliminating all other bleed into the microphone. Then adjust the release to allow the full length of the snare to pass through, so it still sounds natural. You can then focus your EQ on the snare drum, without making the cymbals more present in the mix.
The range control is how much the gate will reduce the overall volume when closed, using a lower range will make the gate more transparent. A higher range will allow for complete isolation, but can sometimes sound unnatural.