Peak reverse voltage or simply means how high the voltage can be over a reverse biased diode. For instance, over a 1N4148 signal diode VRRM can be no higher than 100V. If you apply a higher voltage the junction will be damaged due to zener or avalanche effect. Two ratings who are important for a rectifier are IF (forward current) and VR (reverse voltage). As a non native English speaker I hope I've made myself clear.