What is Gain?
Microphones measure sound waves through tiny differences in voltage between its diaphragm and back plate (See How Condenser Microphones Work) These differences need to be amplified to an audible level and we do this through Gain Control.
This adds more electrical energy to the signal.
On your mixer recorder this should be set so the average speaking level of your subject so your metering is PEAKING around the orange part of your display. (Remember too much red = dead, as distortion will happen)
You are setting your gain in line with the average signal you are receiving from the source for optimum signal to noise ratio and overall sound quality. In the case of speech from a person, get them to deliver a few lines or just chat to them and adjust. Make compensations on your gain control to ensure enough headroom (Space before 0dB) in case of any louder sections from the speech such as unexpected laughter.
Hopefully this explains and answers What is Gain? If not leave a comment below with any questions or confusion.