Down fill refers to the soft, fluffy insulation made from the undercoating of birds, primarily geese and ducks. This natural material is known for its excellent warmth-to-weight ratio, making it a popular choice for items like jackets, comforters, and sleeping bags. The unique structure of down clusters traps air, providing effective insulation while remaining lightweight. Down fill is often rated by its fill power, which measures the loft and insulating ability; higher fill power indicates better insulation. However, it is essential to consider that down fill may require special care and can lose its insulating properties when wet.