Gain is a measure of how loudly the sound plays back. You can change gain, just like you can turn the volume up or down on your TV or stereo.
A change of gain over a period of time is known as a fade (for example, a fade-in can take place at the beginning of a piece or at the introduction of a new sound). If a change in gain is done carefully, it can make a great difference to the way in which someone hears and understands your composition.
