The 85th percentile speed is the actual speed of the 85th percent vehicle in the data set which is sorted by speed.
In other words, if we have (say) 1053 vehicles in the data set. The 85th percent vehicle would be vehicle #895 (1053*.85).
The software will sort all the vehicles in the data set in increasing order by speed and pick the actual speed of vehicle #895 in this sorted list. This is the 85th percentile speed value.
The reason this speed is important is that studies have shown that this is the safest speed that drivers will self select when given an opportunity to pick their speeds on any given road. Many jurisdictions set the road speed limit at or very close to this value.