MOSFET
A MOSFET is the tiny electrical switch that almost every modern chip is built from — billions of them on a fingernail of silicon. Think of it as a tap on a pipe of electric current: current wants to flow from the 'source' terminal to the 'drain', but only if you turn the tap. The handle is a third terminal called the 'gate', and the trick is that the gate is separated from the current by a sliver of insulator, so it controls the flow without ever touching it.
Put a voltage on that insulated gate and its electric field pulls a thin layer of charge into the silicon just beneath it, opening a conducting 'channel' between source and drain; remove the voltage and the channel vanishes. An n-type (NMOS) device switches on with a positive gate voltage and carries electrons; a p-type (PMOS) is its mirror image, switching on when the gate goes low and carrying positive charges instead. Pair one of each and you get a CMOS inverter — the logic gate, now scaled to a few nanometres, behind every processor and memory you own.
MOSFET stands for Metal-Oxide-Semiconductor Field-Effect Transistor — a name frozen in the 1960s, when the gate really was metal over silicon dioxide. Modern gates are typically polysilicon or metal over a high-k dielectric like hafnium oxide, so two-thirds of the acronym is now historical. A common confusion: the gate draws almost no steady current (it's insulated), but switching it still costs energy, because you must charge and discharge that gate capacitor billions of times a second — which is where much of a chip's dynamic power goes.