Fungi, those many formed things that pop up in so of the strangest places, usually in the lawn where you didn't really want them... but don't panic. They are a normal part of the ecology to the soil, in fact they are a prime member of the soil life web. What we call mushrooms are in fact the fruits of fungi. Fungi form strands that can be inches to many yards long, a single fungi can cover thousands of square feet. They can form near, on or even in plants.
The roles of fungi in the garden are many fold. They are the prime decomposers of lignin and cellulose of woody materials. They release acids that break down minerals making them available in the soil. They remove minerals from the organic matter and mineral particles in the soil and transport it to plant roots. They also store Calcium and phosphorus. They bind soil particles and small aggregates into larger aggregates improving soil structure, draining and habitat.
Some Fungi form symbiotic relations with plants that benefit both plant and the fungi. The fungi supplies the plant with resources that the plant can't access (water and minerals), in return the plant supplies simple sugars, complex carbohydrates and proteins. Basically fungi form extended root systems for the plants that they associate with, provide habitat for bacteria and protect the plant from pathogens, pollutants and herbivores.
[rant] Another reason to NOT use glyphosate based herbicides is the it kills fungi not just plants. It's like using a nuclear bomb to kill a mouse in your house, sure it kills the mouse but also the dog, cat and you.[/rant]