Coconut coir is the ground up husk from coconuts, and makes an excellent base for DIY potting soil and seed starting mixes. Its fibers hold water like a sponge, while also allowing for drainage. Because it does not have any nutrition, use it as a base for your mix. Add compost, worm castings, and/or fertilizer, plus perlite, pumice or sand for extra drainage. See the video below for how to use coco coir to make your own potting soil!
Coir also makes a great addition to your planting beds to help improve soil structure, opening up heavy clay, and adding water retention to poor sandy soils.
Coir is an excellent alternative to the environmentally destructive peat moss that is often called for in potting soil recipes. Peat is mined from ancient bogs in a process that is akin to logging old-growth forests. The bogs regenerate slowly and are being destroyed (mostly for potting soil) much faster than they can grow. The bogs also sequester carbon, which is released in large quantities when the bogs are mined. Use coir (or anything else really -- ground bark, compost, leaf mold, etc) in your potting mixes, and check the labels before you buy potting mix at the nursery!