Radar reflectivity is a measure of how much of the radar's transmitted energy is scattered back to the antenna from targets in the beam, and it depends on the number, size, phase, and shape of the raindrops, hailstones, snowflakes, or other scatterers filling that sample volume.
What the radar is measuring
A weather radar emits a pulse of microwave energy, and a fraction of that pulse is scattered back to the antenna by targets in the atmosphere. The strength of the return depends heavily on the sixth power of the target diameter, so a small number of large drops or stones returns much more energy than a very large number of tiny cloud droplets. That physical bias is what makes reflectivity a useful proxy for precipitation intensity while also making it easy to misread.
The returned power is compressed onto a logarithmic decibel scale relative to a standard target and reported as dBZ. Light rain and drizzle typically fall below 20 dBZ. Steady showers land in the 30s. Cores of intense convection climb through the 40s and 50s, and reflectivity above roughly 60 dBZ is almost always dominated by hail, because raindrops that would produce that much return would break apart before they could form.
How forecasters and spotters use it
The first read is location and organization. Where is precipitation, how is it arranged, is a cell isolated or embedded, is a line bowing, is a hook wrapped around the back of a supercell. Structure on reflectivity carries most of the storm-mode diagnosis before any velocity or dual-polarization product is consulted.
The second read is intensity and depth. Cores above about 50 dBZ that extend well above the environmental freezing level indicate strong updrafts holding large ice aloft, which is the classic radar footprint of a hail producer. A tall, narrow reflectivity core is a signature of a durable updraft. A shallow core that leans sharply downshear can mark a weakening or outflow-dominated storm.
Important limits
Reflectivity is a bulk property of everything inside the sampled volume, and that volume grows with distance from the radar. At long ranges the beam sits high above the surface, so a strong echo aloft may weaken considerably before reaching the ground, or the ground may be receiving heavier precipitation than the elevated beam suggests.
The sixth-power weighting also means a few large melting stones can dominate the return, which is why hail can be misread as extremely heavy rain when only reflectivity is used. Bright-band contamination near the melting layer, ground clutter, biological targets, and anomalous propagation can all inflate reflectivity where no meaningful precipitation is present. Reading reflectivity honestly means always asking what the beam is actually seeing at that range and height, and pairing it with velocity and dual-polarization data before drawing a conclusion.