These rockwool gro blocks are used with hydroponic setups. This is a sterile growing medium for the roots to grow into. It is made from finely spun glass, so it has a high surface area, allowing water to be easily absorbed into the roots and also giving lots of air to prevent root rot.  Rockwool is great for propagating cuttings because they have a sterile medium. Plants can be fully grown in these gro-blocks