From The Aquarium Wiki
Rockwool or mineral wool is a fibre made out of molten stone that is spun into dense, thin strands.
It is used in the aquarium hobby wrapped around the roots of young plants by commercial plant growers like Tropica as it is an excellent medium to hold water, bacteria and oxygen. It holds and protects the delicate roots from harm and supplies the roots with nutrients from the water whilst it is grown for sale.
When the end-user buys a rockwool wrapped plant it is necessary for the owner to remove much of the wool as it may hinder the long term growth of the plant or cause the plant to rise as the wool may become buoyant.
The wool is easily torn off by hand and the owner should expose as much of the plants roots as possible without tearing the root structure.
- Tip: When setting up a new aquarium keep any rockwool and put it into your filter or leave on the substrate. The nitrifying bacteria in the rockwool will help to speed up the cycling process.