Official Aviation Definition
An Instrument Landing System (ILS) is a precision radio navigation system that provides aircraft with both horizontal (Localizer) and vertical (Glideslope) guidance signals toward a runway touchdown point. Categorized from Cat I to Cat III, it permits safe landings in heavy fog or low cloud ceilings.
In Simple Words
A high-precision electronic guidance system that lets planes land safely even when the pilot cannot see the runway through thick fog.
Real-World Flight Example
The airport upgraded Runway 16L to Category III ILS, allowing aircraft with autoland capability to touch down with zero visibility.