From the website

How to Read the Map:

Referring to the Map Legend at the bottom left corner of the map, locate Monitoring Stations around the country that are contributing radiation data to this map as you read this, and watch the numbers on those monitoring stations update as frequently as every minute (your browser will automatically refresh). The numbers represent radiation Counts per Minute, abbreviated CPM, and under normal conditions, quantify the level of background radiation, i.e. environmental radiation from outer space as well as from the earth's crust and air. Depending on your location, your elevation or altitude, and your model of Geiger counter, this background radiation level might average anywhere from 5 to 60 CPM, and while background radiation levels are random, it would be unusual for those levels to exceed 100 CPM. Thus, the "Alert Level" for the National Radiation Map is 100 CPM, so if you see any Monitoring Stations with CPM value above 100, further indicated by an Alert symbol over those stations, it probably means that some radioactive source above and beyond background radiation is responsible.

[link to www.radiationnetwork.com]