in no expert but this is what i understand it to be.
Mixture: fuelflow to engine. also fuel cools the engine down a bit. less mixture= less performance/hotter engine and/or choke it. more mixture=cooler engine, better performance\or drown the engine. its all a matter of keeping the balance
Pitch: increases/decreases engine rpm by trimming the angle of the propellerblades. every engine has max torque/hp @ a certain rpm(performance peak). under or over rpm reduces engine power. by keeping the rpm at the given engine rpm, you allways make sure to get the most out of the engine. again also a matter of balance.
by experimenting with throttle/pitch/mix at different altitudes,speeds,dives,climbs you will soon notice what works best for the different engines. also reading up in the interweb about different plane engines will save you a great deal of time. after i started using adv.eng.controls ive increased my cruise speed in the 109s with 60/100kmh.
also if your starting with experimenting, id suggest turning engine overheat off until you feel you got the hang of it... hope this helps a bit
i cannot explain anything better in detail
as i have no experience with this in IRL