Mycelium is a root-like structure of a Fungi which is made out of small filaments called hyphae. It grows through soil or some other substrates. It allows the fungus to grow, absorb nutrients, and produce Mushrooms.
Mycelium network can be too small to see with a naked eye, or can be so big that its weight is measured in hundreds of metric tons.
Mycelium can form symbiotic relationships with plant roots called Mycorrhizae, which allows fungi to "trade" water and nutrients for carbohydrates. These connections can join multiple separate plants together, forming a Mycorrhizal network that allows exchange of nutrients between all participants.
It can also form parasitic relationships, where nutrients are extracted from living organisms (like Cordyceps does). Other fungi use mycelium to extract nutrients form dead organic matter (Saprobic Fungi).