A coil tap switches a humbucker from double coil (e.g., a Les Paul) to single coil (e.g., a Strat). Provides greater variety of tones. You can have a toggle switch installed on your guitar or pots on your tone or volume knobs that are pulled up for the tap and pushed in for "normal."
It's actually pretty simple to install yourself if you're handy and own a soldering iron. Otherwise, a shop can probably do it for about $40 bucks. The taps cost a couple bucks each and maybe $30 for labor.