The mycelium is the part of a fungus that is usually underground (or inside some another substance). It is composed of hyphae, which look like threads, or rootlets. The mat of hyphae may be very thickly woven. The fungus uses it to extract nutritients. The hyphae are usually syncytic, that is, they do not have complete cell walls. Fungal colonies composed of mycelium are found in and on soil and many other natural materials.
A mycelium may be tiny, forming a colony that is too small to see, or it may be extremely large and cover the floors of a large forest.
Mycelium Facts for Kids. Kiddle Encyclopedia.