Our objective is to measure the resistance offered by human body to an applied DC voltage. The resistance is mainly due to the skin.
The resistance is measured and displayed. The value depends a lot on the wetness of the hand. Increasing the surface area of contact of the wire with the body reduces the resistance. Try by inserting a coin between the wire and your hand.
The result shown in the figure is 1.1 $ M\Omega $.
We are using Ohm’s law to calculate the resistance. The current from PV1 flows through your body in to A2. What you measure in the voltage across the input impedance (1 Meg) of A2. Resistance of the body and the input resistace of A2 forms a potential divider network. For example, if your body offeres 1 MOhm resistance the voltage at A1 will be half of the voltage at PV1.