A Star was an obstacle found in the Battle Rooms that Battle School Army soldiers were forced to maneuver around and find ways to use to their advantage in zero to nil gravity. They were likely held in place magnetically or via gravity control, as they did not respond to physical contact. For any given battle there could have been a vast range of stars present, anything from none to very large numbers clustered randomly, simulating shapes, or evenly spaced, sometimes even giving an obvious advantage to a single army.
Most armies and their commanders found stars to be nothing more than annoyances since their locations were seemingly random and didn't necessarily work well with their pre-planned formations, which most commanders utilized in all situations. However some commanders, like Ender Wiggin, found specific ways to use stars to their advantage by having their soldiers use them as makeshift bunkers that they could use as cover while firing on exposed enemy troops, as hiding places for ambushes, or for concealment of formations as they were constructed and before they were utilized.