A mycorrhiza is an association between a plant’s roots and a fungus. Fungal threads extend into the soil and can help the plant obtain water and mineral nutrients. The fungus, in turn, receives carbon compounds supplied by the plant.
This is a biological exchange, not simply a fungus sitting beside a root. It helps explain why a plant’s surroundings include a living network as well as soil particles. The benefits depend on the species and conditions; the association is not a promise that adding any fungus will improve any plant.