It is simply a potential divider and unity gain buffer amplifier. The voltage at the output of the amplifier is the same as at pin 5 but drawing current from it will have less effect than if you measured pin 5 directly.
The two resistors normally set their center voltage to half the supply so you get 2.5V a the output. When skin resistance is in parallel with the lower resistor, it's value is reduced and the voltage drops. Use Ohms law to calculate the effective value of the lower resistor, (1M * Sr) / (1M + Sr) where Sr is the skin resistance.
Brian.