Mycelium is part of the fungi kingdom and is the network of threads, called hyphae; from which mushrooms grow. Not all mycelia fruit mushrooms, depending on the environmental conditions, but all mushrooms come from mycelia.
Mycelia are most prevalent in fields, forests, and heavily wooded areas. Its hyphae; a network of web-like structures that form mycelium, secrete enzymes to break down food sources to be used by the organism. There are some mycelia that span over thousands of acres and some that are microscopic.