What exactly is a coil tap? How will it enhance my tone and will installing one devalue my guitar?

HUMBUCKERS can be tapped, to provide a single coil tone, sometimes the
wiring for this is installed when the pup was made, often it has to be
added, usually by changing to a multi-condictor cable. And then we need
some way to select this feature, a switch, push-pull pot, etc. Or, if
we're clever, it can be added to the existing tone control.

Probably the mods to the guitar for switching, etc, affect the value
most. If we avoid new switches, etc. then it's only the devaluation of
having non-original wiring. But I sure wouldn't do it to a vintage PAF....