The linear sliding control that adjusts a channel’s output.
A fader is not a volume control, it is a variable attenuator. When the fader is fully down, it is at maximum attenuation, and when it is fully up, it is at minimum attenuation. The signal, therefore, is always present, the fader just determines how much of the signal is allowed to pass through.
This can be seen as similar to a sluice gate in a lock. While the gate is shut or down, no water is allowed to flow. When the gate is raised, the water may flow. Opening the gate further lets more water flow.