Plain English Meaning

It is the corridor of sky around an airway that is checked for terrain and obstacles. As long as you stay inside that corridor and at or above the minimum altitude printed for that segment, you are guaranteed to clear everything below by a safe margin.

Full Definition

The protected airspace along an airway or en route segment within which terrain and obstacle clearance is guaranteed when the aircraft is flown at or above the published minimum en route altitude. It extends a specified distance either side of the airway centerline, with primary and secondary areas, and provides at least 1,000 feet of obstacle clearance in non-mountainous terrain and 2,000 feet in designated mountainous terrain.