the MOSFET as a switch

The most common use of a MOSFET is the simplest: an electronic on/off switch with no moving parts. Drive the gate fully on and the channel becomes a tiny resistance (milliohms), so the device passes load current with almost no voltage drop and almost no heat, like a closed switch. Drive the gate to its source (off) and the channel vanishes, blocking current like an open switch. A 3.3 V or 5 V logic pin, through a suitable logic-level part, can switch amps of motor, lamp, or heater current that the microcontroller could never supply directly.

The everyday low-side wiring is: load from the supply down to the drain, source to ground, and the gate driven by your control signal. When the gate is high the MOSFET pulls the load's bottom end to ground and current flows; when the gate is low the load is disconnected from ground. A pull-down resistor on the gate keeps it off when the controller is unpowered or its pin is floating, so the load does not switch on by accident.

Two practical must-dos. First, drive the gate hard and fully (well above threshold) so the device is deep in the triode region with low on-resistance, not lingering half-on where it overheats. Second, the gate is a capacitor, so to switch fast you must charge and discharge it quickly, which can demand surprising peak current from your drive (a job for a gate driver at high speed or high frequency). Slow gate transitions waste energy as switching loss.