Plain English Meaning
On any given stretch of an airway, this is the lowest altitude you are allowed to fly that still keeps you safely above terrain and obstacles and still lets your navigation equipment receive a usable signal.
Full Definition
The lowest published altitude on an airway segment that guarantees both acceptable navigation signal coverage and required obstacle clearance between the two navigation fixes that define that segment.
Context Anchor
Seen during instrument preflight planning on en route charts when choosing safe altitudes for each part of the route.