Gain is a measure of the ability of an electronic circuit to increase the power or amplitude of a signal. Gain is usually defined as the mean ratio of the signal output of a system to the signal input of the same system.
Commander Worf had Lt. Dax boost the gain on the transporter energizing coils in an attempt to rematerialize Trentin Fala's transporter signal, which had been disrupted by a remat detonator. The gain could not be increased any higher, and Fala died during transport. (DS9: "The Darkness and the Light")