What you heard about is mycorrhizae, complexes formed by beneficial fungi and plant roots.
There are many different ones, and particular plant species form relationships with particular species of fungi.
There are three basic types, depending on where the complex is formed: endomycorrhizae, inside the root hair; ectomycorrhizae, outside the root hair, and endoectomycorrhizae, both inside and outside.
The fungus and the plant have a symbiotic relationship, meaning they help each other to thrive. Fungal cells take in some nutrients and water from the soil more easily than plant cells can, while the plant gives the fungus other resources.
Mycorrhizae have been known for many years, but only recently have plant scientists been able to isolate the strains for certain plants and reproduce them en masse for application.
Commercial mycorrhizae preparations are available for many conifers and broad-leaved trees and for certain annuals and turf. The association forms only in the fall, and the fungus normally needs to be injected into the soil.