Plain English Meaning
A wind sensor in the middle of the airport that shows what the wind is doing across the field, not just at the ends of the runway. If its reading is very different from the wind at the runway threshold, that difference is a warning that the wind is changing sharply across the airport.
Full Definition
A wind-sensing device located near the middle of an airport that reports wind speed and direction representative of conditions over the runway environment as a whole, rather than at a specific runway end. The reading is typically displayed to controllers and may be passed to pilots, especially when wind conditions differ between the threshold sensors and the centerfield sensor — a common indicator of wind shear or microburst activity on the field.