Plain English Meaning
A special fast-updating radar that lets controllers safely run two landing streams at the same time on parallel runways that are close together. It refreshes quickly enough that if one aircraft starts drifting toward the other approach, the controller can catch it and turn that aircraft away before the two get too close.
Full Definition
A high-update-rate radar surveillance system used by air traffic control to monitor independent simultaneous instrument approaches to closely spaced parallel runways. The PRM updates aircraft position much faster than standard radar and displays a no-transgression zone between the paths, allowing controllers to and respond to any aircraft drifting toward the adjacent approach in time to issue a instruction.