This post is going to get into expanders and noise gates. If you understand compression, then expanders will be pretty simple. Basically, expansion is the opposite of compression. It's helpful when there's stuff you don't want to come through in the final mix.
Let's look at what that means. A compressor will squeeze audio signals, so that the parts that are quiet become louder. An expander will do just what it says, expand the audio. This will make the quiet parts even quieter. Also, like a limiter being a more powerful compressor, a noise gate is a big expander. The noise gate opens and closes to let sound through, which can prevent more unwanted sounds from getting into the mix.
Now, let's look at some of the common controls found on an expander unit. The ratio knob decides how much to reduce the signal. So, let's say you pick a 1:3 setting. This simply means that for every 1dB the audio goes below the threshold setting, the signal will be reduced by 3dBs. The threshold knob controls what level the expander will reduce the signal at. Everything above the threshold will be unaltered, while audio below it will be reduced by the ratio that is selected. The range control tells the expander the highest amount of signal reduction to allow.
After going through this, the expander can now help you get rid of those unwanted sounds, and bring down some of the mic bleed on something like a snare drum mic. Bring up an expander plug-in and try it out.