Coconut fibers are made from ground up coconut husks. They’re all natural, hold water well, have some root stimulating hormones, and have been found to be a very good medium when mixed 50/50 with clay pellets. A lot of hydroponic gardeners swear by them. They’re quickly becoming one of the most popular mediums for growing just about any type of plant.
One caution, though: with coconut fibers, you get what you pay for. The cheaper ones have been found to be mixed with sea salt to bulk them out. Pay a little more for a better product. Your plants will thank you.