system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: the difference between volume and gain.
gpt
At the simplest level: gain goes in, volume comes out.

Volume is easy.  It's just how loud the sound that comes out of your speakers is.

Gain is how loud the sound that goes *into* the system is, before it's processed by anything inside (like amplifiers etc.).  Increasing gain can therefore increase volume, like how a loud noise simply is louder than a quiet noise at a given volume level.  But they're two independent things, and you could possibly turn gain way up but keep the sound output low by turning volume down.

This could be useful if, say, you have a mic and a keyboard both plugged into a speaker system.  Imagine the mic is kind of broken and isn't very sensitive, and it comes out of the speaker as very quiet.  If you just turn up the speaker volume, then the mic might be at a good volume, but now the keyboard is super loud.  So what you could do is turn up the mic gain to increase the loudness of the sound the mic provides to the speaker without affecting the sound provided by the keyboard.

Turning gain up high typically causes some distortion because you hit the limits of the system.  We know that sound travels in waves; imagine a simple voltage wave that just goes 1-2-3-4-5-4-3-2-1.  Let's say we decide to apply a voltage gain of 5; this means everything get multiplied by 5, so now it's 5-10-15-20-25-20-15-10-5.  But if our system has a maximum voltage of 15, nothing can go above that, so it just becomes 5-10-15-15-15-15-15-10-5.  The sound is louder, yes, but we've lost some definition, and that changes how it sounds.  (That's obviously a huge simplification but conceptually it's still sound [pun not intended lol].)

This is generally used with guitar amps to manufacture distortion.  Amps typically have a gain and a volume knob.  If you turn up the volume knob only, the sound from your guitar just gets louder.  If you turn up the gain knob only, the sound gets louder but also more distorted.  Using both knobs lets you control how distorted your audio sounds *and* also how loud it is; you could turn up gain a little but volume a lot to get a loud sound with minimal distortion.

**Actual ELI5:** take your phone, start recording (whether by taking a video or using an actual audio recording app) and speak normally into it, then play the audio back.  How loud you spoke could be considered 'gain'; it's how loud the *input* is.  You can control how loud the *output* is by using the buttons on your phone; that's volume.

Now start recording again and scream the same thing you said earlier but at the top of your lungs.  Play the audio back.  Notice how it's a lot harder to make out the words you're saying?  That's because your input--the gain--was so high that you hit the limitations of the system (what your phone can process), so you lost some audio definition and got some distorted audio as a result.  You can make it less loud by adjusting the output volume with the buttons, but the high gain has already affected the sound quality.