A transistor is often described as a 'current' amplifier.
It's more accurate to say that a transistor is as 'current-increment' amplifier.
You wire up a transistor in a circuit in such a way that there's a small current
flowing through the input terminals and a larger current flowing through the
output terminals, and then you sit and wait.
Whenever the current at the input changes slightly, the current at the output
changes by a lot more. And that's it.
Doesn't sound like much, but it's big. Really big. It makes big signals out of
little ones. It revolutionized electronics when it was invented around 1948, and
it eventually made calculators, computers, pocket radios, boomboxes, digital
wristwatches, LCD TVs, communications satellites, and smartphones possible.
One transistor all by itself isn't very smart. But they don't dissipate much heat,
so they can be made very small, and a great number of them can be built into
a tiny space. When many of them are arranged in a circuit and built into a small
package that you can hold in your hand, that's called an "integrated circuit", or
sometimes "a chip". They can all be wired up to work together and do smart things.
I don't know the actual number, but there are several MILLION microscopic
transistors in one smartphone !