What is the Master Gain in the Main Control tab?

The Master Gain sets the overall gain of the stereo output. We recommend leaving this control at 0.0dB unless you experience level differences when engaging the plugin.  In those cases, the Gain control can be used to offset any differences in volume level. 

The white circle is your Master Gain control handle, and you can add or reduce overall gain by moving the dot up and down in the Master Output Meter. 

You will see at the bottom of the meter the value of the Gain adjustment that you have made, in this example a -23.7dB gain reduction.