These gourmet mushrooms are wood-loving species, meaning they don't grow in dirt or soil. Normally these mushrooms would be found in nature growing off of trees. To simulate that environment, sawdust is used as a nutritious substrate for mushroom cultivation.
Mycelium is the vegetative part of mushrooms. It's the biomass that feeds on waste and creates a network of fibrous matter. Growing and expanding this mycelium is one of the first steps in mushroom cultivation but instead of dirty soil, you need an extremely clean sterile environment for success.