It's for pilots. At a glance, they can see the flight conditions at any given area.
Each circle represents a reporting point. Each circle is color coded to tell you which "flight rules" apply.
A green circle means VFR means (visual flight rules), meaning that the ceiling (cloud base) is at least 3000 feet AGL (above ground level) and visibility is at least 5 miles. Student pilots and all pilots who are not instrument rated can fly in these conditions.
MVFR means marginal VFR, conditions are "iffy", ceiling 1000 - 3000 ft. AGL, visibility 3-5 miles.
Flying in conditions less than this requires that a pilot be instrument rated, meaning that they have the training necessary to fly just by using their instrument gauges and don't need to rely on what they see out of the windows. All airline pilots are instrument rated!
IFR (instrument flight rules): ceiling 500 - 1000 ft. AGL, visibility 1-3 miles.
LIFR (low instrument flight rules): ceiling below 500 ft. AGL and/or visibility less than 1 mile. (fog would be in this category).