A light trail was a trail of opaque and non-harmful light left behind in the air by certain types of emitters. Most if not all Blastswords left behind light trails, as well as certain lightsabers and other energy based devices when moved at high enough speeds, or when specifically designed to do so.
Light trails from Blastswords were frequently used for comic purposes by New Republic pilot Wes Janson, who took a fancy to the weapon while on a diplomatic mission to Adumar.