On the composite radar image and the base reflectivity radar, the scale on the right is in DBZ, which is a measure of how strong the radar reflection is which is being returned by the cloud.
They also have two different "modes" for weather radar, a so called clear air mode which is very sensitive (can pickup heavy smoke, blowing dust, large swarms of bugs or flocks of birds).
The radar is usually in this mode if you see no echos distant from the radar site and a lot of clutter near the radar.
In the "precip" mode it takes a stronger reflection to show up on the radar plot.
Quick and dirty guide:
As a rule of thumb (as a storm chaser) don't worry much about reflections below 35 dbz, those can be just puffy clouds with no significant rain potential.
35 - 45 dbz you are starting to get some rain potential but mostly light rain
45 -55 dbz potential for strong rain
over 55 dbz probably a strong thunderstorm, 65 dbz storms are big bad thunderstorms that can dump inches of rain, or heavy hail.
http://www.crh.noaa.gov/mkx/?n=using-radarhttp://www.srh.noaa.gov/srh/jetstream/doppler/baserefl.htm