An inventive fellow at the Gibson factory realised that if you built a pickup with two coils of wire, wound in opposite directions, any noise induced on them by stray electro-magnetic fields (like radio stations and unshielded AC power transformers), would be out-of-phase, and cancel out.

The magnets for each coil are also opposite to each other, so that the signals caused by string movement are in-phase, so they add to each other.

End result... the 'good' signal caused by the strings is strong. The 'bad' signal caused by unwanted sources is weak, relative to the 'good' signal. When the strings are being played, the strong signal is way louder than any background noise, so you don't hear anything but the music. When the strings are not being played, the background noise is very quiet, and usually isn't noticeable.

With single-coil pickups, there is no cancellation of stray signals. The noise can be significant, especially when the signal from the strings isn't strong enough to cover it up (like when you're not actually playing).

I'm sure Gibson execs kick themselves for not having the foresight to patent the word 'humbucker', but they didn't, so we have a very useful one-word description for a widely used pickup design.