Pilots typically make turns in the air by banking the aircraft. The act of banking the aircraft is done by moving ailerons. Since moving one aileron up and the other aileron down produces a roll in the direction of the up aileron, the aircraft will bank (and presumably turn) in the direction of the up aileron.
Induced drag is a byproduct of lift. By putting one aileron up and one aileron down, you are producing more lift on the down aileron side and less lift on the up aileron side. Therefore you are also producing a lot more drag on the down aileron side than you are on the up aileron side. This causes adverse yaw in the direction of the down aileron. So, while the bank is trying to fly the aircraft in one direction, the induced drag is trying to yaw the aircraft in the opposite direction. This is one type of adverse yaw.
While turning is done by banking through rolling instead of simply yawing, yawing must still be done. The main job of the rudder is to combat adverse yaw. Pilots are constantly making judicious use of the rudders to keep the flight of the aircraft coordinated and streamlined. Changes in pitch, roll, and airspeed can all cause adverse yaw.
Some more modern planes will do this to some degree for the pilot through electronics. The mechanism for technology to accomplish this is called the yaw damper. This, however, does not completely eliminate the need for rudder inputs like your “mix” setting does.