Mushroom mycelium is the complex network of the fungal organism that colonizes the soil beneath our feet. Think of mycelium as the root or plant while the mushroom itself is the fruit. Mycelium is the longest living and most resilient part of the mushroom's life cycle and essentially acts as the fungus's immune system.
Mycelium is essentially nature's internet and the immune system of the underground. It connects plants and allows them to communicate and even share nutrients through its web-like filaments.
Most plants in the world have a symbiotic relationship with mycelium as they depend on each other for survival. Plant roots use mycelium as protectors against bacteria and other pathogens while mycelium is able to absorb sugars from the plant's roots. Without mycelium, it is safe to say we would have no plants or life on Earth.