A MOSFET can act as a voltage-controlled switch by moving between cutoff and a strongly conducting state.

For an n-channel device used as a low-side switch, a gate voltage below threshold turns the channel off. Driving the gate sufficiently above the source turns it on and allows current to flow from drain to source through a low effective resistance.

The gate draws little steady DC current, but it has capacitance. Changing state therefore requires charging or discharging the gate, so switching speed depends on gate charge and driver impedance.

A real conducting MOSFET dissipates approximately

$$P\approx I_D^2R_{DS(on)}$$

when resistive conduction dominates. During transitions, appreciable voltage and current can coexist and produce additional switching loss.

Switch design therefore involves more than satisfying $V_{GS}>V_{TH}$: the gate must be driven far enough for the required current, device voltage/current limits must be respected, and switching transitions must be fast enough for the intended application.