Plain English Meaning
The area around a ground navigation station where its signal can actually be received and trusted for navigating. Outside that area, the signal is too weak or unreliable to use.
Full Definition
The volume of airspace within which the radio signal from a ground-based navigation aid (such as a VOR, NDB, or DME) is strong enough and reliable enough to be used for navigation. Coverage is limited by the transmitter's power, the type of facility, the receiver's altitude above the station, terrain, and line-of-sight .