the threshold voltage
Every enhancement MOSFET has a tipping point: the gate-to-source voltage at which a conducting channel first appears. Below this threshold voltage, called Vth or Vgs(th), the device is essentially off and almost no current flows from drain to source. Above it, a channel forms and the device begins to conduct, more strongly the further above threshold you go. It is the doorway voltage: too little and the door stays shut, enough and it swings open.
For a common N-channel part Vth might be around 2 to 4 volts, while logic-level parts are designed for a Vth near 1 to 2 volts so a 3.3 V or 5 V microcontroller can drive them. A crucial subtlety: at exactly the threshold the device is barely on, conducting a trickle. To turn a switch FULLY on with low resistance you must drive the gate well past threshold, often to 10 V (or to the part's rated drive). Driving a gate to only just above Vth leaves the device half-on, hot, and lossy.
Two honest cautions. First, Vth is not a sharp constant: it varies from part to part and drifts down as the device heats up, which can cause a hot MOSFET to turn on a little more (an effect designers must watch). Second, Vth is specified at a tiny test current, so it marks where conduction begins, not where the device is a good switch. Always read the on-resistance curve at your actual gate voltage, not just the threshold number.
A part with Vth = 2 V barely conducts at a 2 V gate, but its on-resistance falls from ohms to milliohms as you raise the gate from 2 V toward 10 V: same part, very different switch.
Threshold is where conduction starts, not where the switch is good.
Driving a gate to just above Vth is a classic mistake: the device sits half-on and overheats. Drive the gate fully (often to 10 V) for a real switch.