Fibres of synthetic material, usually polyester, are made into mats or clusters of differing thickness and density. These are used to create insulation that can provide a wide variety of properties.
High loft insulation has larger amounts of air trapped between fewer fibres, saving weight and packed bulk, but holding those fibres apart requires extra processing and technology from the manufacturer. Denser mats of insulating fibres are used where robustness is required rather than low bulk and can even add to windproofness and water resistance.
Synthetic fibres have the advantage of being water repellent and will dry relatively easily after getting wet. Some synthetic fillings are also treated to further increase their resistance to moisture.