A toadstool is a wild mushroom — the kind growing by itself in woods or grass, with a round cap on a thin stem — and the word carries a warning inside it: when English speakers say toadstool rather than mushroom, they usually mean do not eat that. This is an everyday distinction, not a scientific one. Biologists see no difference at all between a mushroom and a toadstool; ordinary people simply use 'mushroom' for the safe ones on the plate and 'toadstool' for the suspicious ones in the ground. In children's stories the toadstool is almost always the red one with white spots, and fairies sit on it, so the word feels storybook-ish and slightly magical to most speakers. Adults use it too, but in a careful way, telling a child not to touch. If you actually want to talk about a poisonous species with an expert, they will call it a poisonous mushroom, not a toadstool.