Plain English Meaning
It is the lowest altitude you are allowed to fly along an airway when you are using satellite navigation. At this altitude you are guaranteed to clear terrain and obstacles, and your satellite-based system will work properly along that route.
Full Definition
The lowest published altitude on a defined airway segment that provides acceptable navigation signal coverage when using a GNSS-based navigation system, while also meeting required obstacle clearance. It applies only to aircraft navigating by GNSS and may be lower than the conventional MEA for the same segment because GNSS does not depend on ground-based VOR or NDB reception.