Plain English Meaning
When the radar that approach control normally uses goes out of service, CENRAP lets them borrow the signal from the bigger en route radar so they can keep working traffic without losing radar coverage.
Full Definition
A backup system that allows a TRACON (terminal radar approach control) facility to continue providing radar services by feeding it long-range radar data from the nearby Air Route Traffic Control Center (ARTCC). It is used when the TRACON's own short-range terminal radar is unavailable or degraded. The center radar data is processed through the terminal facility's ARTS (Automated Radar Terminal ) so controllers see traffic on their displays.