Plain English Meaning
The lowest height you are allowed to fly along an RNAV route, set so you stay safely above terrain and obstacles and your navigation system still works reliably along that leg.
Full Definition
The lowest published altitude on an RNAV (area navigation) airway or route segment that ensures both adequate obstacle clearance and acceptable navigation signal coverage for RNAV-equipped aircraft along that segment. It is depicted on en route charts with the suffix 'G' (for GNSS) next to the altitude value, indicating the route is based on GPS or other rather than -based .