Because of the low electrical signal that a microphone produces, the signal must be “gained” up. A Gain nob increases a signal’s level to normalize.
Important: Gain affects the foundational level of a channel, impacting everywhere it sends the channel from that point on.
Some audio consoles have a "trim" function, which allows for a second level of gain control. Trim does not affect Direct Outs, but usually affects everything else down the line of that channel.