Humbucker

The humbucker guitar pickup was invented shortly after the single coil pickup. The idea was to try and reduce the background "hum" from the single coil pickup that is created from the magnetic field of the magnets. Stacking to single coil pickups next to each other neutralizes this hum.

Some humbucker pickups have metal covers over the top of them to hide the view of the two coils but some do not (second picture below).

The humbucker is a favourite of the heavy metal and rock crowd as they give a heavier sound than the single coil and allow extreme amplification without riding the sonic wave of noises you get from the single coil pickups.

For more information about what the humbucker is check out:

https://www.seymourduncan.com/blog/latest-updates/what-is-a-humbucker