What defines LCD display constrast?
In LCD display,the contrast is about the voltage,you supply for the V0 pin.Assume that the maximum voltage that the V0 tolerate is 5V.And also assume that you have a variable resistance.Now at lower resistance,the voltage drop across the resistance is small,so large amount of contrast is obtained.For high resistance in the variable resistance,the voltage drop at the variable resistance is large,hence the voltage at V0 is appears to be low.So you get less contrast.It's all about ohm's law with the variable resistanceThe voltage at the V0 pin affect the contrast.The voltage is now affected by the variable resistanceResistance is directly  proportional to voltage and voltage is directly proportional to the contrast