rockwool is an artificial material which is used widely here, usually by more experienced growers
It absorbs water like spaghnum moss and it never breaks down(rots)
Its most popular use is in Phragmepedium mixes since they love moisture and dislike sitting in dry soil at any time, I bought a Phragmepedium Don Wimber which is growing in pure rockwool
However, it can be used in mixes for most orchids