Long filler is filler leaf laid or folded so that it runs from the foot toward the head. It is the standard in the premium handmade cigars this encyclopedia covers: whole leaves, or large pieces of them, not chopped scraps.
When it is done well, long filler helps give an even burn and a firm ash, and it lets the blender control the balance of volado, seco, and ligero. It takes more time and costs more to bunch than short filler. Compare mixed filler and the machine-made process in handmade vs machine-made.