The fungi and the trees are in a mutually beneficial relationship: the fungi cannot photosynthesize, as they have no access to light and no chlorophyll. So they get a type of sugar produced in photosynthesis and carbon from the trees.
In return for sugar and carbon, fungi release nutrients such as phosphorous and nitrogen, as well as water, to the trees. It's a win-win.
Furthermore, the fungi will connect one tree to another through their network, which allows them to defend themselves more effectively.
Trees that get attacked by bugs, for instance, release chemical signals into the fungi. Neighboring trees pick up these signals and increase their own resistance to the threat.
Simard presents this as "tree-to-tree" communication. Botanist Stephen Woodward from the University of Aberdeen, however, looks at it not as a dialogue but as the neighboring tree's mere adaptation to survival.