system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly happens when you turn the volume knob on your car stereo? How does the stereo make the speakers produce more sound?
gpt
The amplitude (volume, kinda) of the signal coming directly out of your car's radio, CD player, etc. isn't strong enough to drive much sound out of your car's speakers directly.  First it has to be sent to an amplifier to take the relatively weak signal and increase it.  That stronger signal out of the amp is what goes to the speakers. The amount of amplification is variable...it is determined by your volume knob's setting.  
 
In the past, when you turned your volume knob it turned a variable resistor in an analog circuit that made your amplifier do more or less increasing of that weak signal into a stronger one.  On a lot of newer units, the volume knob changes sends pulses that are counted by a digital circuit, and that circuit then sets the amount of amplification (up to some limit).