You may have spawned in the C172 or in another general aviation aircraft and saw this weird red lever which you can pull or push. You may have also realized that if you pull it all the way, the engines stop running, I am going to try to explain you this function.
Mixture control, to keep it simple, controls the ratio (mix) of the weight of the fuel and the wight of air that is going to be burned. When you move the knob, a little bar is going to block the fuel or the air flow. (See image below)
This is used at higher altitudes where the air pressure is low therefore, you have to move the knob so there is more air flow and less fuel flow.
Here is an image that briefly shows this function:
I hope that you now know what mixture is and does.