what are these readings?
A voltage level on one of the analog pins.
are they skin resistance values?
Not directly but they depend on the skin resistance value if the sensor is applied correctly.
and what code necessary to convert them to skin conductance values? (Micro_Siemens)
You need to calibrate the sensor. The Grove sensor is not designed to deliver µS values. It's thought to show changes in the skin conductance and not to provide absolute values.