A first step in most sound recordings is usually a volume boost; hence the first part of the “Mixing Essentials” on the following topic. As opposed to just ‘turning up’ an entire volume, compressors are used – sometimes already between the instrument and the recording device. The compressor raises the volume of the weaker parts of the sound. This is because what can sometimes be heard well by the human ear in real space tends to be drowned out on the electrified signal path; and even more so in the interaction with other audio tracks.

KEYS author Holger Steinbrink explains a few basics here:

Philipp Ernst from abmischenlernen.de can also be consulted on the topic:

Rick Beato on more specific aspects:

As mentioned here in passing, compression goes from a strong flattening to clear upper limiting of the amplitude for which you use a limiter. At MixbusTV the difference is explained:

Joe Gilder once again summarizes basic functions and recommendations for the use of compressors in a different way. It is about the factors attack and release, about the application to single tracks or several in a mix bus, order of equalizer and compressor as well as the question of volume vs. sound quality.