Plain English Meaning
It's the main navigation source that an instrument approach is built around. If that source isn't working or your aircraft can't receive it, you can't fly the approach.
Full Definition
The navigation aid that defines a specific segment of an instrument approach procedure and provides the lateral guidance the pilot must follow on that segment. On a published approach, the primary NAVAID is the facility (such as a VOR, NDB, localizer, or a designated GPS waypoint) on which the final approach course, intermediate segment, or missed approach is built. Aircraft equipment must be able to receive and use the primary NAVAID, and the facility must be , for the to be flown.