For questions regarding amplitude dynamic-range compression of signals, typically the audio of a music performance.
Amplitude dynamic-range compressors multiply the signal's amplitude by a given value (between 0 and 1), once the amplitude surpasses the given threshold point. Like manually lowering the volume knob of a radio when music gets too loud, but manually is not accurate and fast enough for many applications, so it is done by machines instead, completely automated by the compressor's parameters (attack, decay, threshold, ratio, mix, knee, etc).
If the signal is audio, one common application is to make the louder parts quieter, without touching the already quieter parts, decreasing the difference between the louder and quieter parts. They have many other applications in audio and music.