The Wikipedia entry is a half decent start. http://en.wikipedia.org/wiki/White_space_%...mmunications%29
Basically wireless devices be it cell phones, wireless internet, wireless microphones, TV and radio all have a frequency footprint. The FCC, among other institutions, manage the wireless spectrum so devices don't interfere with each other. With the expansion in wireless devices, especially of late HDTV transmission, there has been a push to open more frequencies for use.
A test like this took place in the past week on Broadway to see the impact on wireless microphones. Looks like it was the NFL's turn. The NFL uses a great deal of wireless: headsets for coaches, TV crews as well as embedded in QBs helmets. The FCC needs to ensure introducing new devices doesn't create havoc in current applications. Sounds like it didn't go so well.