Gain stage refers to any point in a production where you can control the volume level of a device or track. Gain staging is the process of managing all of these volume levels within a project for the purpose of achieving the cleanest mix possible. Gain staging is the process of managing the relative levels in each step of an audio signal flow to prevent introduction of noise and distortion.
Ideal gain staging occurs when each component in an audio signal flow is receiving and transmitting signal in the optimum region of its dynamic range.
In an audio system containing both microphones and loudspeakers, the total amount of gain in the system can exceed 100 dB. This is usually broken up into a number of smaller steps, called gain stages, where the signal is amplified or attenuated as needed before reaching the loudspeaker.