A fancy word for coconut shells
Coir is usually sold in a compressed brick form that expands with water. One drawback - it can be high in salts because of the processing methods used to make it. So it should be soaked in water and washed before being added to the mix.
Coir is currently at the center of the "Great Peat Debate" - being hailed as the environmentally preferable alternative to harvesting natural peat reserves. (Hence the claim "peat-free" in most ads for Coir.) But some have argued that importing organics to the temperate zone from the tropics is also environmentally sketchy. So the debate continues.