Answer: Mycorrhizal fungi are a type of soil fungus that lives on & in the roots of some trees. They help the associated tree absorb moisture & nutrients from the soil and the trees feed them sugars. In some trees or forested areas they literally link trees by their roots into a kind of "rootnet" The theory on adding them to newly planted trees is that they may not naturally be present in the planting soil and that adding them will enhance the success of the new tree.
You can purchase mycorhizzae innoculant from several web sources. It retails for about $70 a pound.
Q&A Library Searching Tips