You're right these progress bars are not documented and actually not really usefull.
They represent the progression of the download of online tiles (images). There are 3 download threads, so 3 bars.
I sometimes use them to see the progress when the internet access is slow and the map doesn't appear quite fast.
Note that is you use offline AQM maps, these bars are not visible.
It is good be able to see the progress but it would be even better to know judge it exactly
I completely agree
I'll try to add a caption or something like that.