A symbiotic relationship is a mutual association between two organisms in which they benefit each other. Such a symbiotic association with plants and fungi is a Mycorrhizal association.
How plants benefit from mychorrizae
What happens in a Mycorrhizal association is that the beneficial fungi attach to the roots of the plants to get nutrition in the form of glucose while the plant benefits by improved uptake of nutrients from the soil using these fungi.
ProMix HP makes an excellent ready to use professional growing medium containing Mychorrizae and you can see how effective it is in growing plants in this video: