Kapok is a natural fiber, similar to cotton. It comes from the fruit of the kapok tree (Ceiba pentandra), which is native to tropical environments. Kapok is extremely buoyant and for many years was used in life preservers.
I've always kept some natural kapok with my dubbing supplies. When I'm tying large, bushy patterns for use on rough water, I regularly use it as an underbody material. Kapok is naturally water resistant and immensely improves the floatability of most any fly pattern.