Sidechain
The Sidechain of a compressor is used to control ‘how the compressor responds to the incoming signal.’
A Sidechains purpose is to allow the compressor to act as if it were behaving in response to an EQ’d version of the sound you’re treating.
This is most often used to remove low frequencies from the incoming signal that’s used to trigger the compressor behavior. It’s useful since bass frequencies tend to cause compressors to respond more sluggishly and inconsistently.
Using the Sidechain to remove low frequencies makes our compressor behave more precisely and predictably.