Gain is the amount by which an audio signal is boosted or attenuated. It appears at many points in the signal path, such as recording inputs, plugin inputs and outputs, and channel strips, and it is usually shown in dB. It is not just about making what you hear louder; its role is to pass a signal at an appropriate level to the next stage.
How it differs from a fader
Gain sets the signal level going into and out of processing, while a fader mainly decides the balance of levels within the mix. Pulling a fader down does not solve the problem of too hot a signal arriving at a plugin earlier in the chain.
Gain staging
Keeping the signal neither too loud nor too quiet at each stage from recording to final output is called gain staging. Leaving enough headroom lets you avoid clipping while adjusting plugins predictably.
Input gain when recording
Input gain sets how strong the signal coming from a microphone or an instrument is before it is recorded. Set it so that the meter does not pin at the top even at the loudest part of the performance. If you record too quietly and then raise the level a lot afterwards, the noise stands out too.
Balancing before and after plugins
Compressors and distortion effects react differently depending on the input level. Set how hard they work with the gain before them, and match the perceived loudness before and after with the output gain, so that changes in tone are easier to judge. The article on ducking with a sidechain (Japanese) is a concrete example.
Points to watch when adjusting
Louder can simply seem better, so match the levels before and after when you compare. Look beyond the red clip indicator and check that you are not sending too hot a signal into the next stage.
