"HUD" stands for "Heads-Up Display". The term is improperly used in Supreme Ruler to describe the unit data icons displayed below unit pictures located on a hex of the game map.
The SR HUD generally consist of an identifying region flag along-side a series of normally green "health" bars, one for each unit located in a particular hex. These green health bars will fade to red as its associated unit loses its effectiveness due to damage, low fuel or low ammunitiion.