Fungi always grow on a nutrient-rich substrate, e.g. soil, trees or food. The fungal fruiting body is the part of the fungus that emerges from the subsoil and serves mainly to spread the fungal spores. In other words, it is the "mushroom" that is searched for in the forest. Many of them can be eaten, but some are inedible due to poison production. However, the fruiting body is only the tip of the iceberg, the main part of the organism is subterranean, called fungal mycelium.
Explanation for children:
Fungi always grow on the surface of something, e.g. on the ground, on trees, on food or on other living things. The fungal fruiting body is the part that protrudes from the surface, also known as "mushroom". It serves to spread the fungus. Many of them can be eaten, but some are poisonous.