from wikipedia:
dBZ stands for decibels of Z. It is a meteorological measure of equivalent reflectivity (Z) of a radar signal reflected off a remote object.[1] The reference level for Z is 1 mm6 m-3, which is equal to 1 μm3. It is related to the number of drops per unit volume and the sixth power of drop diameter.
Reflectivity of a cloud is dependent on the number and type of hydrometeors, which includes rain, snow, and hail, and the hydrometeors size. A large number of small hydrometeors will reflect the same as one large hydrometeor. The signal returned to the radar will be equivalent in both situations so a group of small hydrometeors is virtually indistinguishable from one large hydrometeor on the resulting radar image.
A meteorologist can determine the difference between one large hydrometeor and a group of small hydrometeors as well as the type of hydrometeor through knowledge of local weather condition contexts.