MOSFET switching devices operate in the on and off states. In the “on” state, the impedance of the switch is theoretically zero and no power is dissipated in the switch no matter how much current is flowing through it. In the “off” state, the impedance of the switch is theoretically infinite, therefore no current is flowing and no power is dissipated.
Thedrain-source on-resistance (RDS(on)) is the effective resistance between the drain and the source of a MOSFET when it’s in the on state. This occurs when a specific gate-to-source voltage (VGS) is applied. In general, as the VGS increases, the on-resistance decreases. The lower the MOSFET on-resistance, the better because a low resistance reduces undesired power dissipation, improving the power efficiency of the device.