Plain English Meaning
Marked areas near the runway antennas that broadcast the ILS signal. When weather is bad and aircraft are flying ILS approaches, nothing is allowed to sit or move through these areas because objects in the wrong spot can bend the signal and give the landing pilot wrong information.
Full Definition
Protected zones on or near the airport surface around the localizer and glideslope antennas of an Instrument Landing System (ILS) where vehicles, aircraft, and other objects are restricted during low-visibility operations because their presence can distort the ILS signals and cause the cockpit indications to be inaccurate.