R_ON is the ON resistance of the switch. Remember that the NMOS and PMOS transistors are in-parallel. This means when the switch is on, add the two resistances in-parallel, making a lower combined resistance. The relationship of V_CC and threshold voltage (V_TH) impacts the linearity of the signal through the switch. For example, the 74LVC2G3157 has a strong R_ON peak for low V_CC. However, in devices with an internal charge pump like the CB3Q3257, the R_ON curve follows that of the NMOS device. The charge pump keeps the gate voltage higher so that the switch can pass signals that are even higher than V_CC.