Plain English Meaning

An ILS antenna doesn't only send its guidance signal straight down the runway. It also sends weaker signals off to the side or at the wrong vertical angle. If a pilot picks one of these up by mistake, the cockpit needles can show 'on course' or 'on glidepath' even though the airplane is in the wrong place.

Full Definition

Additional signal lobes radiated by an ILS localizer or glide slope antenna, outside the protected approach area, that can produce on-course or on-glidepath indications when the aircraft is not actually aligned with the published approach path. They appear at angles other than the intended course or glide slope and can mislead pilots who intercept the ILS outside its protected .