I really don't know I suppose they are identifiers for the weather broadcasting station, maybe so the pilot can dial in weather for that location as they get closer??
Not all of them have the information displayed but what I see is the 4 letter station call sign, then a time stamp in zulu followed by what appears to be the FM broadcasting ?wave length. I'm just guessing but that is what it looks like.
Can you give a particular site we can all look at and will attempt to discuss question. (Comm Pilot 40 years)
BTW, this is a great tool. without even scrolling over you can see sky condition (+) clear; dark blue dot - overcast etc. Arrow plus cumulative feathers equal wind direction and approx speed (1 full feather - 10 knots 1/2 feather 5 knots)
Just click on one of the dots or + symbols and a small window should pop up giving you the current temp, relative humidity, wind etc. and below that will be some writing in script that begins with the same 4 letter call sign as displayed on the top of the pop up window.
Best way to direct you to a pop up: click on Denver Centenniel Airport and it will give the information being discussed. I would cut and paste but it won't let me.