logic gate
A logic gate is a tiny circuit, built from just a handful of transistors, that takes in one or more signals and produces a single yes-or-no answer. It treats a high voltage as 'true' and a low voltage as 'false,' then computes one simple rule of logic — what mathematicians call a Boolean function. Think of a bouncer at a door: an AND gate lets you through only if both conditions are met, an OR gate if at least one is, and a NOT gate simply flips the answer it's given.
Each gate does almost nothing on its own, but that's the magic. Wire the output of one into the input of the next, stack thousands and then billions of them, and you can add numbers, compare them, store them, and follow instructions. A modern chip is just an enormous city of these gates switching on and off. Everything your computer does — every game, message, and photo — is built from this one humble idea, repeated at staggering scale.
One gate is special: NAND (AND followed by NOT). It's called 'universal' because you can build every other gate — AND, OR, NOT, all of them — out of nothing but NAND gates. (NOR works the same way.) So in principle, a whole computer could be made from this single kind of part, repeated endlessly.