Plain English Meaning
The lowest altitude you are allowed to fly on this leg of an IFR route. At or above this altitude, you are guaranteed to clear the terrain and obstacles below, and your navigation radios will receive a usable signal the whole way to the next fix.
Full Definition
The lowest published altitude on an airway, route segment, or other IFR route that guarantees both acceptable navigation signal coverage and required obstacle clearance between the two fixes that bound the segment.
Context Anchor
Seen on IFR en route charts along published route segments, especially when planning or flying an route.