Plain English Meaning
How strongly rain, hail, or snow bounces the radar signal back. Bigger drops and more of them bounce more energy back, which shows up as brighter or more intense colors on the radar picture.
Full Definition
A measure of how strongly a target — typically precipitation particles — returns radar energy back to the radar antenna. In weather radar, reflectivity depends mainly on the size, number, type, and phase (liquid or ice) of the precipitation particles within a given volume of the atmosphere. Higher reflectivity values indicate larger or more numerous precipitation particles and are commonly displayed as more intense colors on a radar image.