An ion trail was a detectable byproduct created by starship sublight drives or ion drives. It could be tracked by sensors to determine the direction a ship had traveled. Ships that were equipped with a cloaking device were able to mask this trail, either by dissipating it or storing engine emissions to be released at a later point.
Behind the scenes
Ion trails are used in most space sci-fi genres to various effects. Star Wars ships can only locate a direction of travel, while Star Trek ships can sometimes identify a class of vessel or which race produced the vessel simply by the ion trail it produces.