Make quiet parts quieter on a ratio. Below the threshold, every decibel down becomes several decibels down, which pushes room tone, hum and bleed away from the material you are keeping without the on-off edge a gate has.
Threshold and ratio
The threshold, from -80 dB to 0 dB, is the line the expander works under. Anything above it comes through untouched.
The ratio is how much steeper the signal falls below that line. At 4:1, a passage sitting 6 dB under the threshold comes out 18 dB further down, because each decibel of the original 6 becomes 4. At 1:1 nothing happens at all and the step passes the file straight through.
Knee
The knee, from 0 dB to 24 dB, is how wide a region around the threshold the ratio phases in over.
A hard knee at 0 dB switches to the full ratio the moment the level crosses the line, which is precise and audible. Six to twelve decibels of knee spreads the transition out so material hovering near the threshold is not repeatedly caught and released. Speech, where breaths and word endings sit right at the boundary, almost always wants the wider setting.
Attack, release and range
Attack, from 0.5 ms to 100 ms, is how fast the expander opens back up when the level rises. Short values keep the front of a word intact. Long values shave it.
Release, from 10 ms to 2 s, is how fast it closes again. Short releases follow the material closely and can flutter on sustained quiet passages. Long releases hold the gain open across gaps, which sounds smoother but lets more of the noise through.
Range is the floor. It caps the total reduction between 0 dB and -60 dB, so the ratio cannot ask for more attenuation than you allow.
Expander vs gate vs denoiser
Three steps solve overlapping problems and choosing wrongly is the usual mistake.
Use the noise gate when the noise is far below the signal and you want the gaps genuinely silent. Use the expander when you want the gaps quieter but still alive, or when the noise is close enough to the signal that a gate would chatter. Use the denoiser when the noise is present underneath the signal as well as between it, because neither of the other two can remove something while the material it sits under is playing.
Example: a spoken take in a live room
Threshold -38 dB, ratio 2:1, knee 9 dB, attack 3 ms, release 250 ms, range -18 dB.
The room drops about 18 dB in the gaps and stops there, so the recording still sounds like a room rather than a series of clips. The wide knee keeps breaths from being caught halfway.