To add to mj's post above:
"Unity Gain" means if the level of an audio signal comes into the console at -12dB, and all the console faders are set to zero, then the level of the audio signal as it exits the console will also be -12dB, as though the signal had never passed through the console.
In addition to the faders, changing from neutral any parameter (EQ, Pan, FX, etc.) in the signal path may change the level of the audio signal as it passes through the console.